Enable support for TLS

Closes: #90 (closed)

Testing:

1. Create a self signed certificate for localhost and place the certificate and key in /tmp/server.crt and /tmp/server.key.

2. Create a config file config/gitlab-exporter.yml with following contents

   server:
     name: webrick
     listen_address: localhost
     listen_port: 8443
     tls_enabled: true
     tls_cert_path: /tmp/server.crt
     tls_key_path: /tmp/server.key

3. Run bin/gitlab-exporter web -c config/gitlab-exporter.yml and see that WEBRick is started without errors.

4. Confirm that gitlab-exporter loads all intermediate certificates using openssl s_client -showcerts -connect localhost:8443 < /dev/null

5. Try curl http://localhost:8443 and see that gitlab-exporter doesn't support connecting over HTTP