Add "includePath" option to enable LESS parser to look in more places.

Usage:

  project/
    my_less.less ('@import "import.less";)
    imports/
      import.less ('b { color: gold; ')

Then,

  recess --includePath=imports --compile my_less.less

And the import diretive in my_less.less will find import.less
automatically. Very useful when working with Bootstrap, since you can
add Bootstrap's less/ directory to `includePath` and only override the
parts you want to change (LESS still looks in the "input" directory
first).

More than one directory can be specified:

  recess --includePath=imports --includePath=exports my_less.less

Or, in `.recessrc`:

  {
    'includePath': ['imports', 'exports']
  }
Current version of recess will merge identical declarations.

But, compiled test results had not been updated to address this, in
zero-units.css
Merge pull request #64 from vicvicvic/master

Add "includePath" option (à la lessc)