Highlight arguments

[nicoespeon]

Follow-up #797

Arguments passed into the function are highlighted differently from local variables or global identifiers.

Variations:

• Carry it through to aliases (if we assign the argument to another value, highlight that too)
• Highlight local variables only
• Highlight values that will be assigned to something
• Highlight variables used in loops

[nicoespeon]

In fact, not necessary. You can already highlight what you want and it's more flexible than this would be.