Issue 26 in xmobar: Add a Run Once for executing external commands

[xmobar at googlecode.com]

Comment #2 on issue 26 by jaortega: Add a Run Once for executing external  
commands
http://code.google.com/p/xmobar/issues/detail?id=26

I've implemented this feature in the git repo by letting the
refresh rate to be 0 (or a negative value) to denote "run once"
(instead of introducing new syntax). Thus, now one can write

    Run Com "uname" ["-s", "-r"] "" 0

and uname will be run just once.