Add string/double Fluent Dictionary Option

* For the MultiMatchQuery, this adds an override to OnFieldsWithBoost to
allow one to specify a list of fields and boost values with string
values instead of fluent values.

This is useful in the case of multi-fields where you have no fluent
equivalent of the field name you want to query and boost on.
Merge pull request #964 from edwinf/add-string-double-fd-multimatch

Add string/double Fluent Dictionary Option to MultiMatchQuery