Raw expression support

[tobrun]

Feedback from a user trying out expressions: It would have been nice being able to use raw expressions as used in the style-spec. This would allow for both using the current DSL approach in combination with approach taken in the style-spec.

eg.

iconImage(get("key"))

vs

iconImage("[\"get\", key]")

[jfirebaugh]

How would you distinguish a literal string value from an expression string?

[anandthakker]

@tobrun what about adding a raw() function to the DSL?

iconImage(raw("[\"get\", key]"))

[1ec5]

This will significantly reduce the amount of work needed for nitaliano/react-native-mapbox-gl#1047, since the iOS map SDK already has something like this.

Note that a similar conversion would be needed for filters as well.

/cc @nitaliano