Adding a new switch, --only-generate. This will only create the search.json, won't upload it.

[bsclifton]

This is a minimal version of #3

[bsclifton]

@LeonB, just some context; we're re-working some of the Ember.js documentation and would like to use this plugin. I've been testing the new flag added by this (originally added by @Jberlinsky) and it's working great. Let me know if I can do anything to expedite acceptance of this PR 😄 I've tried to keep it extremely simple

[trek]

👍 this lets us move forward, but we still need to polish towards what originally we spec'ed out. Things still missing or done in a way that needs cleaup:

• search.json is an array. It needs to be a key of documents that holds the array.
• "provide a swiftype:generate command that outputs a search.json file" A thor subcommand is probably the more idiomatic way structure this public API. A cli Middleman plugin (like rake, capistrano and other parts of the ruby ecosystem) tends to group plugins instead of switching on flags
• "search.json file goes into the build directory on middleman build and middleman server". Right now you need to run the command separately instead of it just happening when you build or server.
• "tests must exist around all this behavior to avoid future breakage". We still need tests a around the generation of the search.json file