Added a watch command that will continually poll for changes and rebuild them.
Merge branch 'upstream-master'
1. Added the WATCH_SERVER flag that is enabled by default
1. Added an argument to watch, --no-server which will disable the embedded server.
1. Added a deprecation message to the preview mode
* Preview now shows the deprecation message and calls watch.
* Details on how to disable the server in the command description.
Merge pull request #177 from stoltene2/master

Adding a watch command