Tabs, effect tokens, MD strikethrough, xi-unicode 0.3

This PR contains several changes/improvements:

FormattableText now has an effect_tokens method; also its associated type's name changed
Horizontal tabs (\t) are now supported, though imperfectly (at least, they aren't highlighted when selected)
Markdown supports strikethrough and hard/soft breaks
Updated xi-unicode and use to check for explicit breaks at line's end

kas-gui / kas-text