Add autocompletions for tags & titles in elfeed-search buffer.

[SqrtMinusOne]

This adds notmuch-like autocompletions in the *elfeed-search* buffer. I've seen some discussion in #32 a few years ago, but didn't find anything like that implemented.

It works for:

• +... - tags
• -... - tags
• =... - titles

That's a subset of the possible filter syntax but in my opinion much better than nothing.

The variable elfeed-search-completion-enabled enables autocompletion. elfeed-search-completion-cache enables caching. I've started using elfeed rather recently, so I am not sure how fast would it be to fetch tags and titles from DB at every request. Therefore it's optional and enabled by default.

Built-in completing-read and completing-read-multiple from crm.el are used, so the completion should work with any popular completion framework.