Added ability to deploy to multiple custom domains in VHOST.

As discussed in #263, this PR implements the following changes:
- nginx-vhost plugin now no longer manages `/home/git/$APP/VHOST`
  file but rather reads from it.
- nginx-vhost plugin supports multiple custom domains in VHOST.
- Default nginx configuration template pulled out into separate
  files instead of adding as a HEREDOC in `post-deploy`.
- Added helper messages during deploy.
- Minor code cleanup.
- NOTE: Setting the `URL` for multiple VHOSTs needs refactoring.