Added quasi class for es6 template literal placeholders

[EngineeredTruth]

This is my favorite syntax theme so I updated it to support ES6 template literal place holders.

Check out image for before and after:

[SebastianSzturo]

Thanks for your contribution!
What do you think about using the same colours we use for ruby string interpolation?

I will take a closer look on the weekend.

[EngineeredTruth]

I like the blue since the theme doesn't use that color as often. It makes it easier to distinct it from other things, but I can make it the same color as ruby string interpolation for the public repo!