Bump async

makes 2 1/2 changes

1. adds a bump command which increments each of the packages by the specified amount, major, minor, patch, and prerelease are supported, if no field is present it defaults to '0.0.0' so that doing bump minor would give you 0.1.0, also adds tests and updates the readme. 

2. makes it async, currently the program reads a file, does the stuff, writes it, and then looks to the next file, used the async file methods so file 2 doesn't have to wait around for file 1 to finish writing before it can start, should be faster (through who knows by how much).

Also I fixed a typo in the readme.