Initial code for reading fill styles from XFL files.
For now, it can read only solid colors.
When generating an SVG path element, we use the edge's right-side fill style color for the interior of the path.
(Just for now. In a future we'll interpret other fill styles too and generate correct filled shapes.)
Filename of the XFL file can be now specified from the command line.
If the given file cannot be found, an appropriate error message is displayed.
Also, the require paths bug has been corrected by adding the top-level script's path to the `$LOAD_PATH`.