FEATURE: Oauth/SAML based Reverse Proxy Module

As a system owner of a proxy, I would like to optionally integrate an added layer of authentication/authorization to restrict access to only those privileged users/groups. For a GSA system, it makes sense to offer the following:

### Oauth based
- [ ] Google (w/ whitelisted domains - @gsa.gov account only)
- [ ] Github (w/ whitelisted orgs/groups - @gsa or @18f members or specific teams within)

### SAML based (*may be more appropriate to link to another repo as a standalone component*)
- [ ] GSA SecureAuth (using https://shibboleth.net/)
- [ ] Max.gov (using ?)

**Example:**
https://github.com/bitly/oauth2_proxy