Merge with default config

[maxandersen]

Why:

• if you set authtoken using ngrok config add-authtoken and you also set config options in application properites we ignore the authtoken from the default config.

This change addreses the need by:

• use ngrok merge config feature by passing default config location config on the command line (iff the default config exists)

[maxandersen]

https://ngrok.com/docs/agent/config/#config-file-merging is the docs I used to do this.