Suggested changes for token support (#16)

[kenodegard]

The gist of these changes:

• removes ExtraContext, this is redundant with what is aggregated and provided in the ctx (click.Context) object
• adds not_reuqired_if logic to custom option so click will properly handle the error handling (instead of handling it post processing in get_auth_manager)
• rename conda_auth.hooks to conda_auth.plugin per the naming pattern established elsewhere (see Convert conda-build entrypoints into plugins conda/conda-build#4921 (comment))
• add -h support (to match what conda itself supports)

Controversial/opinionated changes that I'm indifferent about:

• inlined minor helper functions that have minimal usage (1-2 calls)

[travishathaway]

LGTM 🚀 Thanks 😄