Command Line Application

As discussed in https://github.com/diffplug/spotless/issues/76, building a binary wrapper for Spotless seems to be a (probably?) necessary step to building a cleanly written Bazel extension. Since much of the work needs to be done anyway, why not make that executable accessible to Spotless users as a command line application?

@nedtwigg has kindly already provided some input on this in this [comment](https://github.com/diffplug/spotless/issues/76#issuecomment-587940340)