Add support for global commands

[jakemac53]

See #198, which would add a mono_repo travis --validate option. Today it is clunky to configure - you have to add it to some random mono_pkg.yml file in the repo and do a cd ../ before the command.

Adding the general ability for repo-level tasks (instead of sub-package tasks) would solve this. Probably an extension to the mono_repo.yml file?