Use a separate face for Julia macro invocations.

Fixes https://github.com/JuliaLang/julia/issues/8721
Highlight quoted symbols, e.g. :foo.

Fixes https://github.com/JuliaLang/julia/issues/8713.

Since we don't have a full parser, we can't always highlight :end
correctly. Distinguishing between:

    foo[:end] # indexing with symbol

and

    foo[bar:end] # using end as a slice index

is not possible without a parser. As a compromise, we treat :end as
always a slice, since that's far more common.