Allow verification of "internal" usernames against LDAP

[tarkatronic]

Right now the "internal" usernames in the config file are purely symbolic. They don't actually do anything except act as an indicator of sorts. For example:

[members]
jwilhelm = "tarkatronic"

The jwilhelm value is unused, where as the "tarkatronic" value is a GitHub username.

The intent here is that the jwilhelm should represent an actual "internal" user, and it should be possible to verify that username. Used in the context of a company, this can validate, on every run of the tool, that an employee is still with the company and thus still belongs in the organization.

Ultimately it would be great to have pluggable backends for this but really, I think that LDAP is a great first effort for being able to talk to many different backends.