New formula: redshift

http://jonls.dk/redshift/
This isn't a purely build-time dep:

```
otool -L /usr/local/bin/redshift                                                            [~]
/usr/local/bin/redshift:
	/usr/local/opt/gettext/lib/libintl.8.dylib (compatibility version 10.0.0, current version 10.3.0)
```