Add "includePath" option (à la lessc)

Based on issue #57 – I didn't write any tests because the tests that are there only related to the linting of recess. This uses the same strategy as `lessc`'s `--include-path` but instead allows you to use `--includePath` more than once to specify multiple paths (instead of using path separators).

Also uses the name `includePath` instead of `include-path` because camelcase seems to be the recess-way.

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']
      }