Add import (-I) and export (-E)

This can be useful if you want to save (and later re-apply) tags. Some
filesystems do not support extended attributes (xattr), and in this case
it is wise to make a backup of all the tags on your files.

Right now basic support is in place, I will look at implementing some
--exclude option to exclude certain paths, as well as a way to provide a
``base directory'' for all both import and export.

Right now, you can ``fake'' a relative directory by just running the tag
program from the directorly you want to use a relative base.
Fix import/export; fix clear; add -V for verbose.
Add TODO file.