Subshell parsing fixes

[sc0ttj]

This PR changes the way Bash code can be embedded inside your Markdown..

It used to be this:

some markdown $(your_code) text here

Now, after the changes in this PR, it's this:

some markdown <?bash your_code ;?> text here

Reasons:

• more robust
• much fewer parsing errors
• more verbose/explicit syntax
• slightly less horrific code
• enables future feature: support languages other than Bash, like so:
  • <?python print 'hello world' ;?>
  • <?ruby puts 'hello world' ;?>