Let tools specify parser settings

[fkling]

Currently tools can only specify which parser to use (so it is selected by default) but not parser settings, which is required as well sometimes.

[hzoo]

Ah yeah It would be useful for babel to only run the plugin you are creating (similar to eslint without any rules except the one being created)

[RReverser]

@hzoo That's pretty easy to do even without configs - just need to change couple of lines in current code.

[hzoo]

@RReverser would we want an option for that or just not transform anything else by default?

[RReverser]

@hzoo I believe that should be default as that's usually what user expects (mixing built-in transformation combined with own one is confusing when you're trying to develop a plugin).

[hzoo]

@RReverser Ok can do a PR - #112