[WIP] Fix #39: Generate correct source paths

[lydell]

• Allow options.filename to be a path to the intended output file (not
  just the basename of it).

• Set the file property of the source map to the basename of
  options.filename, or don’t set it at all if options.filename was
  omitted.

• Allow options.sourcemap to be a string: The path to the intended
  location of the source map. If relative, it is relative to the current
  working directory. (Only the dirname of options.sourcemap is used.)

• If options.sourcemap is set to true, assume that the source map is
  intended to either be placed next to options.filename, or be inlined
  into the CSS as a data uri, by using the dirname of options.filename
  or the current working directory if options.filename was omitted
  (instead of using the dirname of options.sourcemap as in the above
  bullet point).

• Make relative source paths relative to the source map directory before
  adding them to the source map.

• Make relative source paths from applied original maps relative to the
  source map directory before adding them to the map.

  TODO: Tests and docs.

[lydell]

I won’t bother to write tests until we have decided if this is the way to go.