add a cucumber feature for passing parameters to includes
add support for parameters to {% include %} tags
document include tag parameters
add unit tests for include tag parameters
fix cucumber feature to obey new date format
fix liquid syntax in the docs
do parameter parsing with @imathis' incredible regex

This also introduces single-quote support.
add single-quote cucumber testing
stupid Ruby 1.8.7 doesn't support named groups
To satisfy Ruby 1.8.7, use look-ahead instead of look-behind
Ruby 1.8.7 only supports 1 param to Regexp.match()
move regex to the top
move parameter parsing to render time
Support passing Liquid variables to includes

Change the regex matching to allow Liquid variables and object fields
to be passed to the include. Use the render context to retrieve the
variable value. Also, relax syntax checks by allowing surrounding spaces
and dashes in parameter names.
Add a cucumber test for passing variables
code improvements according to @parkr's suggestions
more code improvements

Remove unused variable, extract validation to method (@mattr-).
Do not require markup to be passed to parse_params as argument.
set value outside if/else block
Add 'docs' subcommand. Solves #1043.
Merge pull request #1046 from mojombo/jekyll-docs

Add 'docs' subcommand. Solves #1043, solves #1288.