FR: mos-tool can't clone libs with submodules recursively from github

When including a library which contains a git submodule, the submodule will not be cloned/fetched.

Proposal: add a flag ```--lib-recursive``` to the mos-tool and with this flag set, the libraries should be fetched with the submodules as well.

Addendum: perhaps this should be an "opt-out", so recursive is per default active, but might be supressed by a flag like ```--lib-non-recursive```