Add multi-term support to Query#term

[olivernn]

Previously a single term could be passed to the Query#term method, additionally this term needed to be a string. No tokenization is performed by this method. The API was a bit cumbersome when dealing with user entered queries, tokenization had to be done manually, and then the results manually iterated over and used with Query#term.

This change improves matters in two ways:

• Lists of terms are supported
• A term can be anything that responds to toString

Together these two changes make it possible to directly use the lunr.tokenizer when passing strings to Query#term.

[olivernn]

This has now been merged into master and is released in version 2.2.0