Generalize configuration through `monorepo.exs`, adds task `monorepo.changes`
and `monorepo.compile`.

New monorepos should use `repo = "./repo"` in `monorepo.exs` at root level to
establish repository configuration. For now `monorepo.compile` can be used
for a only compiling changed applications in the repo but regular `compile`
should work fine.

There's no significant testing done at this point but two moderately complex
umbrella projects have been converted without any difficulties.