Improve behavior when ngrok is already started

[juherr]

Current behavior:
If ngrok is already started, it won't be started even if the expected port is not listened to.

Expected behavior:

• If the ngrok account accepts multiple sessions and ngrok doesn't listen to the expected port, the startup should start ngrok with the expected port.
• If the ngrok account doesn't accept multiple sessions and ngrok doesn't listen to the expected port, the startup should fail.

[kilmajster]

I left related comment in MR #55 (comment)

[juherr]

The feature request is still valid: the user account could be checked in order to provide a more expected behavior.
But I didn't find a public api for the account.