Add `gts clean` command

[JustinBeckwith]

We should add a command to do clean instead of inserting an rm -rf command into package.json.

[JustinBeckwith]

@ofrobots does the commit in #13 mean that the fix, lint, and clean commands should all be parsing the users local tsconfig.json, and using that for source/dest paths by default?

[ofrobots]

Yes, that was my thinking. That would minimize the number of places where the same path information would be duplicated (and the user will have to keep in sync). We should definitely not have any hard-coded paths embedded in gts.

[JustinBeckwith]

Covered in #15