Auto-completion for tags field

[raucao]

• Cache existing tags in localStorage when all bookmarks are loaded
• Read from localStorage and use with new autocomplete UI when adding or editing a bookmark
• Delete cache key when disconnecting storage

closes #29

[raucao]

I think this is done for the basic functionality. Would appreciate a quick review.

[raucao]

When removing a bookmark with tags that are only used for that bookmark, it might be a good idea to delete those tags from the cache.

They'll be overwritten entirely next time you load the index, hence the todo method also for adding tags.

Also, when adding tags via autocompletion the last trailing comma results in an empty tag being added.

The auto-completion is only putting things in the input field, so apparently there's a bug with trailing commas in general.

[raucao]

I moved the tag list deletion to the service and fixed the bug with trailing slashes. Thanks for your review!