Add negation support to the "Multiple words" parser

Problem/Motivation

No bundled parser currently supports negative keywords (= find stuff NOT containing the keyword). Negative keywords are pretty standard functionality nowadays starting from Google and ending with Drupal core search that supports negation out of the box. Preceding the negative keyword with a minus sign has become a sort of a de-facto standard that most users would expect (and try).
What's important, the Search API backends currently DO support negative keywords, it's just that no bundled parser would parse minus-preceded keywords into correct array structure for the backends to pick up.

Proposed resolution

A keyword preceded with a minus sign becomes a negative keyword, so that the index items containing the word are excluded from search. You can negate quoted multiple-word keys as well (make sure the minus is before the quote).
I hesitated between creating a new parser and adding the functionality to the existing "Multiple words" parser. Yes, theoretically it changes existing behavior but I still decided to modify the existing parser because:
- the functionality is pretty standard and intuitive and should work "out of the box" in my book. Currently it's one of the very few features that work in the core Search and don't in Search API
- to break some existing web site functionality, the site in question needs to 1) contain some meaningful keywords starting with the minus sign where the minus sign is important (can't come up with a single plausible use case, sorry) and 2) be already set up in a pretty unorthodox way as the current implementation of Tokenizer is hard-coded to remove minus signs from both search query and indexes (which is wrong in my book but that's whole 'nother story).

Remaining tasks

Reviews needed, tests probably needed.