Clear Guest Author caches upon Guest Author creation

After a Guest Author is created, we should flush all caches to ensure
any previously cached negative results are gone.

Also need to `$force` the lookup in `delete_guest_author_cache()` to
ensure it never hits a stale negative cache entry.

Fixes #218
Merge pull request #220 from Automattic/fix-218

Clear Guest Author caches upon Guest Author creation
Bring back strict equality checks in guest author lookup caching

The non-strict equality check causes negative cache entries to always
hit the database and re-set themselves.

This reverts commit 6ad24ff7f853f7c6df613456f02051d711456a65.

See #218

Fixes #219
Merge pull request #221 from Automattic/fix-219

Bring back strict equality checks in guest author lookup caching
Test last two versions of WordPress and trunk
Clear all caches for Guest Author on save

Prevents stale negative cache entries
Add better Infinite Scroll support for Guest Authors

Filter the IS settings to remove any author ids and replace them with
`author_name`, so that the standard CAP Guest Authors query filters
work as expected.

Problem existed when a Guest Author slug matched a real user in the
network (not necessarily a member of the site), and WP injects `author`
into the query vars.

IS would pick that up for future requests, which ended up with queries
that CAP didn’t properly understand / fix.

Fixes #229
Added a filter to the Co Authors Open Graph tags.

This allows to disable some particular opentags by filtering the returning array.
Merge pull request #232 from vaurdan/master

Added a filter to the Co Authors Open Graph tags.
Improve CLI commands with more flexible author lookups

Attempts both a lookup on both the unsanitized and sanitized version of
`user_login` when assigning co-authors, for imported content that may
have used a different user system in the past, with different u/n rules.

Also rectifies the username passed to actual add_coauthors() call when
making the assignment. Previously we passed the `$original_author` -
now we pass in the `user_nicename` of the object retrieved from the
database, as it doesn’t necessarily match `$original_author`