New option "output_path" added.

[Harald-LB]

The native library has a function sass_option_set_output_path which is attached to ruby in native_context_api.rb. But in engine.rb this option is not set (why?).
However to produce correct source maps, the output path must be known to the native library.

The use of the parameters :source_map_file, :filename and :output_path is shown in the test_source_map_filenames test in file test/engine_test.rb.

By the way, all these parameters need to be documented. Where is the right place to put this?

[bolandrm]

Thanks for the PR! For parameter documentation, I think it would be good to start a new section under usage.

[brendon]

This is a great addition. Is there any reason why this has stalled? Without this option, source maps where the output file isn't in the same directory as the input file don't appear to be possible.