Don't connect to the remote debugger when using --pre-install

[kumar303]

If the remote debugger connection is broken for unknown reasons (#317), allow the user to specify run --pre-install as to not make a remote debugger connection (as a workaround).

[kumar303]

documented: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/web-ext_command_reference#--pre-install