fix watch to properly pass port variable to browsersync opts

[djeglin]

I have changed lib/watch.js to properly pass the port variable from the command line through to browsersync.

[jescalan]

Looks good! Just a couple things:

• Does this still work well even when there isn't a port option passed in?
• Could you add a test for this option to prevent regression? Let me know if the test structure is confusing 😁

[jescalan]

Beautiful, thank you!