How to: File provider
This provider uses yaml file with routing rules.

`reproxy --file.enabled --file.name=config.yml`

Example of `config.yml`:

```yaml
default: # the same as * (catch-all) server
  - { route: "^/api/svc1/(.*)", dest: "http://127.0.0.1:8080/blah1/$1" }
  - {
      route: "/api/svc3/xyz",
      dest: "http://127.0.0.3:8080/blah3/xyz",
      ping: "http://127.0.0.3:8080/ping",
      remote: "192.168.1.0/24, 127.0.0.1" # optional, restrict access to the route
    }
  - {
      route: "^/admin/(.*)",
      dest: "http://127.0.0.4:8080/$1",
      auth: "admin:$2y$05$..." # optional, per-route basic auth (htpasswd bcrypt format)
    }
srv.example.com:
  - { route: "^/api/svc2/(.*)", dest: "http://127.0.0.2:8080/blah2/$1/abc" }
  - { route: "^/web/", dest: "/var/www", "assets": true }
"*.files.example.com":
  - { route: "^/files/(.*)", dest: "http://123.123.200.200:8080/$host/$1" }
```

This is a dynamic provider and file change will be applied automatically.