Added support for environment variables in config.
Added support for environment variables in config.

New token type, added parser logic.

valdur55/py3status,alexoneill/py3status,tobes/py3status,ultrabug/py3status,guiniol/py3status,guiniol/py3status,valdur55/py3status,tobes/py3status,Andrwe/py3status,valdur55/py3status,Andrwe/py3status,ultrabug/py3status,ultrabug/py3status,vvoland/py3status