Better syntax highlighting for directives

[Herteby]

In Vue-syntax-highlight directives get full javascript syntax highlighting, which I think is much nicer than this one, where they are just shown as strings. Or maybe it'd be better to just take out the language definition, leaving that to vue-syntax-highlight, and only provide the snippets.

[maxkoryukov]

I think, this could be solved in #8

[qkdreyer]

https://github.com/vuejs/vue-syntax-highlight/issues/52#issuecomment-285162761