Read configuration files from outside .go-proxy

[visola]

User Story

As a user, I want to be able to share my upstreams with other working in the same code base.

Acceptance Criteria

• [x] There's a concept of custom folders and it is persisted in the current state
• [x] There's an Admin UI where the user can select folders to be scanned
  • [x] the folders will be scanned every minute for a file called .go-proxy.yml recursively
  • [x] hidden folders are ignored (starting with a .)
  • [x] upstreams found are added to the current list of upstreams
• [ ] Files found get saved in the current state
• [ ] When restarted, upstreams from previous found files get loaded with the ones in the ~/.go-proxy/upstreams directory