Fixes#190.
With this feature unsee can be configured to proxy requests to selected Alertmanager instances, if it's enabled unsee silence form will send a request via unsee rather than directly. This allows users to manage silences in environments where they have access to unsee but not to Alertmanager. Only silences endpoints on Alertmanager API are proxied.
This adds support for reading configuration from file, env support is still there and legacy env variables will still work, but flags are now following config schema, so they don't match old flags.
Having a config file allows to express more complex configuration options, which is needed for some feature requests.
