Add simple completion support for watcher

[spinscale]

This adds basic completion support for watcher. Some things need to be extended,
but it should already help a lot.

Several things are not covered here yet:

• This should only work, when the watcher plugin is actually installed, right now the kibana code on start up only checks the version in es.js, but it might be a good idea to also check the installed plugins and decide based on that what to load. This would be even more useful with all of our open source analyser, repository and discovery plugins.
• Compare condititions, handling of fields with arrays and recursive fields like the various chain stuff are not supported

[spinscale]

@spalger ping, anything I can help with? Adding this conditionally only when watcher is active?