implement -s switch to specify search path

By default dex will look through $XDG_CONFIG_HOME/autostart and
the xdg/autostart/ subdirectories of the $XDG_CONFIG_DIRS.

The -s switch lets you override the built in directory search path with
one of your own.
Merge pull request #19 from kyrias/search-dir

implement -s switch to select directory search path
replace hard-coded colon py os.pathsep
update README and add changelog