Extract the inlined matches-filter-params? function

This was originally inlined using a `(letfn [..] ..)` block as two
functions: `matches-filter-param?` and `matches-filter-params?`.

Pull this out into a separate multi-arity function to make it easier
to read.
Merge pull request #25 from alphagov/extract-matches-filter-params-function

Extract the inlined matches-filter-params? function