This adds functions for getting directories according to the XDG base dir
specification. This might be useful in general.
This also adds a function for getting the cache dir, because I like an explicit
function call more than something which "switches" based on a string argument.
Better error messages if you mis-type something. :-)
Finally, this adds a function for getting the directory containing the rc.lua
file. Sadly, awful.util.getdir("config") did not actually do that. See #218.
Fixes: https://github.com/awesomeWM/awesome/issues/218
Signed-off-by: Uli Schlachter <psychon@znc.in>