should do something useful with git submodules

[disruptek]

[disruptek]

So the tricky bit here is figuring out how and where to specify requirements as we move them into git submodules. The larger goal here, and the reason I moved this into the milestone, would be to eliminate the .nimble files altogether.

The most obvious solution I could come up with would be to overload the name field in the .gitmodules to include the acceptable versions. Maybe we can add some other arbitrary key/value pairs in there, too; I rarely use submodules and I don't really know how much we can hack at this.

Here's an example for reference:

[submodule "bump ~1.8.* & < 2"]
  path = bump
  url = https://github.com/disruptek/bump
[submodule "github ^2.0.3 & < 3.0"]
  path = github
  url = https://github.com/disruptek/github

cc @Clyybber