Unowned team globs

[perryqh]

Addressing #52

Allow for team unowned_globs, which subtracts from their owned_globs.

Example:
config/teams/foo.yml

name: Foo
owned_globs:
  - foo/service/**/*
unowned_globs:
  - foo/service/exclude/me.rs

In this example, team Foo owns all files under foo/service/ except for foo/service/exclude/me.rs

[martinemde]

Thanks for taking this on and adding this feature so quickly. @ivy and I read through it and did our best to understand :bulbasaur: Looks good!