switch to grunt-css-flip for RTL CSS generation
Disable requireCamelCaseOrUpperCaseIdentifiers JSCS option completely, for now

Can't disable it just for the Gruntfile due to https://github.com/mdevils/node-jscs/issues/287
Merge pull request #12949 from twbs/use-our-grunt-plugin

switch to grunt-css-flip for RTL CSS generation
re-enable requireCamelCaseOrUpperCaseIdentifiers in JSCS & use `null` to override it for Gruntfile
Merge pull request #12950 from twbs/jscs-null-trick

re-enable requireCamelCaseOrUpperCaseIdentifiers in JSCS