Add `{view, string()}` changes feed option

This makes it possible to specify a view to be used as filter for the
changes feed when using `couchbeam_changes:fetch/2` or
`couchbeam_changes:stream/3`. Note that CouchDB ignores that filter
parameter as long as the value for the `filter` parameter is not set to
`"_view"`.
Merge pull request #82 from KlausTrainer/master

Add `{view, string()}` changes feed option