Made applications source Mac OS X compatible
Merge pull request #21 from junkblocker/patch-3

Made applications source Mac OS X compatible
allow to change error color through zstyle
zaw-open-file: use `xdg-open` by default, use `open` if $OSTYPE is darwin
zaw-src-ack: hide `--group` option from user

Because `--group` option is only for this source function to parse
output, there is no need to tell user is uses this option internally.
zaw-src-ack: pass `--nocolor` option to ack command

Because ack may output color escape sequence even though stdout is not
terminal at some environment, pass `--nocolor` to surpass that.