Added basePath option and support for relative paths

This PR is primarily to support relative paths using a dot. In order to support them however, neuter needs to know the 'base' path of the file (what is usually added by the filepath transform) so that it can omit it from the dot's replacement value. The simplest and most dev-friendly way to accomplish this is to just add an option that does the typical work of the `filepathTransform` automatically.

Note that this PR doesn't support relative paths without a `./` at the beginning as suggested in #23.