Add support for Gravity.START and Gravity.END values

RTL locales support is a good idea for a lot of production-ready apps. So it would be nice to add more abstract "start" and "end" container gravity values instead of hardcoded "left" and "right".

were you able to support tag views starting from the right side of the screen on RTL interfaces @Bringoff ?

were you able to support tag views starting from the right side of the screen on RTL interfaces?

@royherma well, I've switched to FlexBoxLayout and implemented it manually :)