Integrate doctest with testsets

[davidanthoff]

This issue is a follow up on #774 and #198, and what a nice public API for that functionality might look like.

My own preference would be something like this in my runtests.jl:

using Documenter, Test, MyPackage

@testset "MyPackage" begin

include("test_featureA.jl")

Documenter.run_all_doctests("MyPackage")

end

I.e. ideally I don't really want to deal with any subtleties at all, but would just like to call one function, and have it sort out everything. In terms of testset integration, my preference would be if run_all_doctests started a new (child) testset and then put each doctest into that testset.

The name for the function is not great, this is more an idea how the API should feel like.

[mortenpi]

There are actually two separate ideas here: (1) provide a simplest possible API for runtests.jl, and (2) integrate it with testsets. I'll dedicate this issue for the testset integration and and leave the API discussion for #1051.

Regarding the testset integration: yep, I think it's a good idea. For the time being though, it's up for grabs -- it needs a slightly more sophisticate implementation for doctest.