Optionally match empty values in query parameter presence matcher

[sarayourfriend]

Fixes #110.

This PR simplifies the header and query param existence matches by introducing a new base matcher class, ExistsMatcher. Two new matcher classes, HeaderExistsMatcher and QueryParameterExistsMatcher are also introduced and the existing header(s)_present and param_exists methods are updated to use these matchers.

This removes the need to use the catch-all regexes for the value-check of the existence matchers. The new query-parameter existence matcher can be configured to allow empty values, which fixes #110 while preserving the existing behaviour of requiring some value for the matcher's default behaviour. This prevents breaking backwards compatibility in the default behaviour. Setting allow_empty=True makes it possible to successfully match requests like https://example.com/?refresh where refresh is interpreted as true by the requested API if it is present and false if it is missing or if it is explicitly set to false (refresh=false). Here is a real-world example of such a parameter in a widely used API: https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-reindex.html#docs-reindex-api-query-params.

This backwards compatibility consideration does not effect headers, for whose existence matcher already allowed empty values in all cases because headers were never ignored due to their empty value.