Adds Renderer.text hook for unformatted text.

Useful for building alternative output methods (like docx) using the Renderer class.
Documentation for version 0.3.0
Expose default_features as class-level attributes

This makes it possible to add feature types to BlockLexer and
InlineLexer without having to copy-paste the methods where their default
feature orders are defined.
Format for list
Add benchmark for hoep
Define list_features and footnote_features for BlockLexer, so that one
can custom the parsing process of list and footnote.

Related issues:
- https://github.com/lepture/mistune/issues/8
- https://github.com/lepture/mistune/issues/11
Version bump 0.3.0
Fix block_code render. No need to add `\n` in <code>
Add documentation on custom lexers. An example on GitHub Wiki links.

https://github.com/lepture/mistune/issues/12
https://github.com/lepture/mistune/issues/13
Make hard_wrap a method for InlineGrammar.
Fix errors in readme on lexers
Add tests for custom lexers
You can pass inline and block as a class to `Markdown`.
Trim whitespace in inline code.

https://github.com/lepture/mistune/issues/15
Improve benchmark
Version bump 0.3.1
Use inspect to detect class.

https://github.com/lepture/mistune/commit/3af9c0b2763afe91addd69e483bac6e771815995
Add rtd command in Makefile
Merge branch 'master' of https://github.com/lepture/mistune
Enables alternative representations of the render tree.

This is useful for using an empty list as the default output value instead of a string. This allows renderers to build their own stack of directives using Python objects. This makes it easy to convert that tree of objects into other document formats besides HTML (such as docx).