vim: Expose the search-by-type feature

[Julow]

This adds the :MerlinSearchType command to query for a value with a matching type. It works like the previous :MerlinSearch but uses the new search feature added in https://github.com/ocaml/merlin/pull/1828, the search results are shown in the completion menu.

:MerlinSearch switches between polarity search and search by type depending on the first character of the query, like is done in the emacs plugin.

Documentation about the three search functions is added.