simple propertize support for t-l-buffer-modified segment

[MikeTheGreat]

This is my second try for #95.

I've fixed this up so that it supports propertized-strings, but doesn't have all that customization cruft :)

(If this still isn't cool please feel free to reject it - I'll keep the changes in my personal files, and it was fun figuring out enough git to do this :) )

[dbordak]

This is closer, but I probably want to put a bit more thought into the default theme.

I don't want to make a per-segment face, but i might want to e.g. use a common "good" face between this and the error checker. I'll have to think about this a bit.