Schneems/pre erb

[schneems]

Placing pre.erb in-front of another command will allow dynmaic templating via Ruby's ERB syntax.

For example:

:::>> pre.erb $ echo "The answer to everything is <%= 6*7 %>"

When this runs, it will first replace the template with the result of the ERB. It would be the same as this:

```
:::>> $ echo "The answer to everything is 42"
```

The binding (variable and method scope) for pre.erb is shared across all executions and the default print.erb command. That means you can use it to persist data or logic and re-use it:

```ruby
:::-- print.erb <%
  # Won't be rendered because it's using `--` visibility
  def lol
    "haha"
  end

  user = "Schneems"
%>
```

```
:::>> pre.erb $ echo <%= user %> said <%= lol() %> | tr '[:lower:]' '[:upper:]'
```

When run, this would produce:

```
$ echo Schneems said haha | tr '[:lower:]' '[:upper:]'
SCHNEEMS SAID HAHA
```

Multi-line commands are also supported

```
:::>> pre.erb file.write "lol.txt"
Super secret key:
  <%= "#{key}" %>
```

The only thing to watch out for is if the resulting template contains a :::>> (or similar) rundoc marker at the beginning of the line; Rundoc will think it is a new command rather than a part of pre.erb template.

The visibility of the pre.erb is forwarded to whatever command is run.