Add file including mechanism

[yusiwen]

I've added a mechanism to including external files into _config. By using <!#PATH_TO_FILE> syntax, it can include the content of the external file.

I'm using it to switch i3 color themes. I made a symlink to a color theme file, which only contains i3wm's color settings, and included this symlink in _config. So, next time I change theme, I need only change the symlink to another theme file, and reload i3wm.

BTW, thanks for your great work.

[tckmn]

What is the data variable doing there? It looks like you're creating a variable but never using it.

Also, what's the reasoning for using !#? (Not that I have a better suggestion; just curious. Could be confused as a comment.)