Add ActionIfExists.MERGE_REFS_OR_APPEND

[LeMyst]

Close #607

[LeMyst]

DRAFT...

I'm trying to improve the ActionIfExists enum by using Enum.Flag

Like, you want REPLACE if exist or APPEND otherwise:
action_if_exists= ActionIfExists.REPLACE | ActionIfExists.APPEND

You want to just APPEND new claim to existing property:
action_if_exists= ActionIfExists.APPEND

Same if you just want to REPLACE existing value without doing nothing more:
action_if_exists= ActionIfExists.REPLACE

If you use the word ActionIfExists.KEEP, this do nothing if the property already have claim.

If you want to merge references, you can use the ActionIfExists.MERGE_REFS, in addition with the APPEND and REPLACE
action_if_exists= ActionIfExists.APPEND | ActionIfExists.MERGE_REFS

[dpriskorn]

Very nice work. I just tested it, see dpriskorn/WikidataEurLexScraper#2, and I'm happy :)
LGTM

[dpriskorn]

Hooray!