To-do: update the integration tests accordingly.

Closes: 328

Previously, `configlet generate` assumed that every line beginning
with a '#' character inside a concept `introduction.md` is a header.
For our purposes, the most common exception to that idea is probably
a comment inside a fenced code block. With this commit, we no longer
add a '#' to such a line.

The markdown handling here is rudimentary, but it may be good enough to
avoid a full markdown parser for `configlet generate`.

But this doesn't support other blocks, like HTML blocks.

Also update the tests to assert that `configlet generate` makes no
changes to the current state of the elixir track repo.

In a later PR we'll update those tests to assert the same for
`configlet generate -uy`.

This is arguable. But I think this way is more readable overall by the
time we add handling of HTML comment blocks.

More readable, and easier to extend to support HTML comment blocks, but
probably marginally slower.

This is only one of many possible HTML blocks [1], but probably the most
important one.

[1] https://spec.commonmark.org/0.30/#html-blocks

Insert a h2 using the concept's name in the track `config.json`, and
demote the headers in the `introduction.md` for the `foo` concept by 1:

    # Introduction

    %{concept:foo}

Don't insert a h2, and demote by 1:

    # Introduction

    ## Foo

    %{concept:foo}

Don't insert a h2, and demote by 2:

    # Introduction

    ## Some header

    Blah blah.

    ### Foo

    %{concept:foo}

Consider this `.md.tpl` file:

    # Introduction

    ## Foo

    %{concept:foo}

    ## Bar

    %{concept:bar}

Before this commit, when more than one placeholder had a reference link,
`configlet generate` would produce something like

    # Introduction

    ## Foo

    Here is a line with a link [Foo][foo].

    [foo]: http://www.example.com

    ## Bar

    Here is another line with a link [Bar][bar].

    [bar]: http://www.example.com

With this commit, we place the reference links at the bottom, ordering
by first usage, and deduplicating them:

    # Introduction

    ## Foo

    Here is a line with a link [Foo][foo].

    ## Bar

    Here is another line with a link [Bar][bar].

    [foo]: http://www.example.com
    [bar]: http://www.example.com

But it's tricky to do this robustly without using a proper markdown
parser.

From the CommonMark Spec [1], the rules for reference links have some
complexity:

    A link reference definition consists of a link label, optionally
    preceded by up to three spaces of indentation, followed by a colon (:),
    optional spaces or tabs (including up to one line ending),
    a link destination, optional spaces or tabs (including up to one line
    ending), and an optional link title, which if it is present must be
    separated from the link destination by spaces or tabs. No further
    character may occur.

    ---

    A link label begins with a left bracket ([) and ends with the first
    right bracket (]) that is not backslash-escaped. Between these
    brackets there must be at least one character that is not a space,
    tab, or line ending. Unescaped square bracket characters are not
    allowed inside the opening and closing square brackets of link
    labels. A link label can have at most 999 characters inside the
    square brackets.

[1] https://spec.commonmark.org/0.30/#link-reference-definitions

Clarify the kind of slug. In the future we could even consider:

    type
      ConceptSlug* = distinct string
      ExerciseSlug* = distinct string

The CommonMark spec [1] calls them headings, not headers.

[1] https://spec.commonmark.org/0.30/

Make declaration order match usage order.

The backticks look a bit strange, even if you know that `concept` is a
keyword. Sidestep the issue. We named it `con` elsewhere:

    $ git grep --heading --break 'for .* in concepts:'
    src/info/info.nim
    32:      for con in concepts:

    src/lint/track_config.nim
    303:  for con in concepts:

That's what the CommonMark Spec [1] calls them.

[1] https://spec.commonmark.org/0.30/#link-reference-definitions