Use submodule for QMK source

[rswiernik]

Currently, the QMK source is manually pulled into the repo. This opens up the possibility of errors and issues when copying in the source, as well as the possibility of falling behind of mainline QMK very quickly. From an organizational perspective, I think it would be better to have the QMK source included as a submodule. It's even pretty reasonable to pin this submodule to a specific version tag.

Thoughts? Was this looked at and passed over for some other reason I might be missing?

[drashna]

Considering at this point, the QMK firmware source is almost 2 years out of date...