Add "index" flag to CLI index command

[jakewrfoster]

Overview:

It would be helpful for testing purposes to be able to write data to an index other than the index that is generated via a given site's SearchPress settings/configuration.

Proposed User story:

As a user initiating an index operation from the SearchPress CLI, I should be able to indicate the target index for the index operation.

Proposed MVP:

• Add an "index" flag to the CLI index command

Nice to haves:

• Indication regarding whether or not an index exists with the provided name.