Introduce --ignoreDeleted

[raDiesle]

This is the process how we use reg-cli:

• /path/to/expected-dir folder is git commited
• whenever developer instatiates/installs the current repository ( e.g. yarn install), it will also cleanup temporary build files,
  therefore it will delete all files of /path/to/actual-dir generated from last test run and reg-cli
• developer knows e.g. by CI or the component he touches, which test he needs to run to update and verify image
• when a developer performs a single test, reg-cli will write the images related to this test to /path/to/actual-dir with --update operation,
• this results in situation that git commited /path/to/expected-dir is not just updating the single test run related img diff, but also the non existing images from other tests to be deleted.

Suggested Solution

• add reg-cli option of --ignoreDeleted which will not update non existing images, but only changed ones from actual to expected dir

Alternative is to manual git revert all images which are not related to the test he was running