List markdown improvements

[kopylovvlad]

Hello. I opened markdown guide and compared existing functional. Here the list of not supportable features. Look at the list, please. What do you think about it? Are they all useful for the project? Should we implement theim?

nested lists

1. Item one
2. Item two
3. Item three
    * Sub-item
    * Sub-item
4. Item four

Problem: it raises an error 'Illegal nesting: nesting within plain text is illegal. (Haml::SyntaxError)'

alternative ways of indicating h1 and h2.

This is an h1
=============

This is an h2
-------------

Problem: it raises Haml::SyntaxError

bold and italic by underscore

_This text is in italics._
__This text is in bold.__
___This text is in both.___

Problem: it does not transform the text

Paragraphs

Problem: It does not render paragraphs

Horizontal rules

***
****************

Problem: It does not transform the text

Link with a title

[Click me!](http://test.com/ "Link to Test.com")

Problem: It does not transform the text

Link with relative path

[Go to music](/music/).

Problem: It does not transform the text

Reference style links

[Click this link][link1] for more info about it!
[Also check out this link][foobar] if you want to.

[link1]: http://test.com/ "Cool!"
[foobar]: http://foobar.biz/ "Alright!"

Problem: It does not transform the text