Import existing bookmarks should update tag list by merging the tags lists

[t-w]

Currently there are 2 options for importing existing bookmarks (having the same URL):

• not overwriting - basically any existing bookmark is skipped (time updated)
• overwriting - all data related with the bookmark is overwritten

For me this clearly lacks the 3rd option: import an existing bookmark merging the lists of tags.

This can be implemented in several ways, adding dedicated option(s) etc. But it is better not to complicate (I am 100% for keeping Shaarli small and simple). The easiest way to add this is just to merge the list of tags by default (when not overwriting! - overwriting can be used for deleting / cleaning-up the tag list).

I made a simple change that does this, I will make a PR. The issue is created in case further discussion is needed.