Interpreter: Normalize flags in the arguments before processing them.

This fixes `--option-a=false --option-b=parse --out=compiled.js` and `-abc` (which I haven't seen used in/with traceur).
The examples above would be converted to `--option-a false --option-b parse --out compiled.js` and `-a -b -c`, so the flags are properly recognized.

The normalization is similar to what Commander does internally, but I'm doing it in-place so only traceur arguments get touched by it.