Allow forc index deploy to automagically stop the current version of the indexer

[ra0x3]

• We have the ability to deploy new indexers to an indexer service via forc index deploy
• When this new indexer is deployed, it starts to index data immediately
• However, any previous version of the indexer that has not been explicitly stopped, will continue to run
• Given that indexer migrations feature: index migrations #382 don't really make the most sense given our setup, the main way to "migrate data" is to update your schema, rebuild your indexer, then redeploy
• This is simpler than it seems: you just edit your schema file then do another forc index deploy
• However, in order to ensure that users can deploy this new indexer without having to go back and manually stop an older version, we should add some flag to forc index deploy that will automagically stop any indexer running under that namespace/identifier combination
• Something like forc index deploy --stop-previous would upload the new indexer and stop any previous version
  • Call the flag whatever you want that makes sense

[ra0x3]

@rvmelkonian Looks like @deekerno has already done this #640. I think this might be good to close.