Add support for line numbers in code blocks

[allejo]

Summary

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed issues	Closes #105

Description

Adds the ability to start highlighting specific lines in code blocks. Here's the implemented syntax.

• lang - Don't highlight any lines
• lang{4} - Highlight just line 4
• lang{4-6} - Highlight the range of lines from 4 to 6 (inclusive)
• lang{1,5} - Highlight just lines 1 and 5 on their own
• lang{1-3,5} - Highlight 1 through 3 and then 5 on its own
• lang{5,7,2-3} - The order of lines don't matter
  • However, specifying 3-2 will not work.

This also adds the new highlighter.line_numbers configuration file option that will allow users to opt-in to displaying line numbers for all code blocks.

This feature has been added to both the Markdown and RST engines.

Check List

• Added appropriate PhpDoc for modifications
• Added unit test to ensure fix works as intended