These commit includes changes to start the server as one of admin (also
IMAP) or phish server. Before this change the servers used to run in
monolith this change will decouple the two core component i.e. admin
and phish server so that they can be run independently.
This will help where admin and phish server are required to run
saperately e.g. phish server runs in a DMZ.
The available modes are `admin`, `phish` and `all`. Running the binary
in the `admin` mode will start the admin and IMAP server, while running
the binary in the `phish` mode will start the phish server. `all` mode,
which is also the default mode will start admin, IMAP and phish servers.
e.g. `go run gophish.go --mode admin`