MathJax support, second attempt

[jmendeth]

Based on #113. The differences are:

• The interface now gives a displaymode boolean instead of the raw delimiters. The renderer must struggle with the process of escaping math code as appropiate.
• When HOEDOWN_EXT_MATH_DOLLAR is not active, $$ math $$ inside a paragraph gets rendered inline instead of block.

[jmendeth]

1. Okay, so now the extension guesses from the context. If the only contents in the parsed snippet are the math span (and optional spacing around it) then it's block. Otherwise, inline.

2. There's a gotcha: it tests the snippet, not the whole block. So in edge cases like this one (with quotes enabled):

   There's some math here: "$$x$$"
   

   The quote extension will parse the quote span, and call parse_inline with the snippet $$x$$. Thus parse_math will render it as a block. In that case it has to be forced to inline:

   There's some math here: "\\(x\\)"
   

3. I've renamed the additional flag to HOEDOWN_EXT_MATH_EXPLICIT. Why? Because that flag is incompatible in both ways (it screws existing documents when enabled, and when disabled). So I think the new name reflects that better.

[jmendeth]

Example:

Some $$inline$$ math here.

$$ block math $$

gives

<p>Some \(inline\) math here.</p>

<p>\[ block math \]</p>

But there's one missing thing: HTML renderer needs to escape delimiters. I don't know how to handle this.

@uranusjr how should snippets like: $$\[x\] = x$$ render?

[uranusjr]

Using \[, \], \(, or \) inside a math block results in a syntax error in TeX, so IMO we can just treat it like an undefined behaviour and Blame The User™. I think it would be perfectly reasonable to simply do nothing and output \[\[x\] = x\].

[jmendeth]

Allright, then.

[jmendeth]

@devinus This is ready to merge for me.