Use comma as "or" operator for queries

[tomjaspers]

Scan the query parts for commas (u',') and treat these as (sub) AndQuerys
that need to be wrapped in an OrQuery (if there are more than one).

Supports foo, bar and foo , bar.

See #976

[brunal]

That means that any query holding a comma should now be escaped... I'm not sure how I feel about that.

[sampsyo]

Awesome! This looks great to me. As that thread in #976 suggested, a comma seems like the right thing to me. It's not quite true, @brunal, that every comma will need to be escaped—commas are only ORs when they appear by themselves or at the end of a term. So foo, bar is an OR query but foo,bar is not. In the shell, quotes even behave as expected, so:

$ beet ls "foo, bar"

does not invoke an OR query.

I'm inclined to merge this now. We could restrict this to only work with separated commas (foo , bar), but I'm inclined to say that this is less surprising as-is.

[tomjaspers]

@SampSo My original implementation only worked with separated commas, but I found myself constantly typing foo, bar during testing, so for ease of use I'd definitely try and keep it as-is (if possible).

[sampsyo]

OK, merged! We can at least see how people like this design. Thanks again! This will be awesome to have. ✨

[raffomania]

Awesome! Thanks a lot, @tomjaspers!