Support markup to include the contents of a file

[kminder]

I'd like to be able to break a large document up into sections and then have a master document that imports all of the sections. The master document would then look something like this:

Title
=====

Table Of Contents
-----------------
* [Some Section](#Some+Section)

<<section.md>>

[jirutka]

I think that the cleanest solution for this is to implement a preprocessor that will merge the included sections with the main document and then pass to the Pegdown.

[kminder]

You are probably right. In fact this is exactly what I have done in a very naive way my project. I thought the idea might be interesting to be included in PegDown somehow. In particular a command line processor that used the preprocessor would be very useful in many situations. Most importantly some discussion/consensus about what the markup should be would be very valuable. I don't remember how I ended up settling on using << >> in my preprocessor. In any case it needs to mix well with the rest of the Markdown markup.