Add a widget for multiselect facets with configurable operand

[olanowak]

Do you plan to add support for multi-select facets?
Here is a little example of what I would like to accomplish:

title:
[x] a (1111)

• b (222)
• c (333)
  [x] d (4)

author:
[x] a (123123)
[x] b (123)

• c

After checking facets (marked as [x]) I would like the query to look like this:
(title:a OR title:d) AND (author:a OR author:b)

I would like not checked facets to not disappear and that all of them have appropriate counts.
I tried to use James McKinney tips from this question https://groups.google.com/forum/#!msg/ajax-solr/F8f6f3BtlUE/i3kdQYg_wzEJ but the constructed query looks like
/solr/select?rows=25&facet=true&facet.field={!ex=title}title&facet.field={!ex=author}author&fq={!tag=title}title:a&fq={!tag=title}title:d&fq={!tag=author}author:a&fq={!tag=author}author:b
so the operator is AND and I don't know how to change it to OR. I'm stuck.

It would be great if there was multi-select widget with configurable operand so users wouldn't have to change something withing Your code.