Match on composite MIME types as well

[fluffy-critter]

If a template is designated as having a MIME type of foo/bar then it should also match an Accept: of foo/baz+bar if there are no direct matches.

For example, with this template mapping:

{
    '.byaml': 'application/blap+yaml',
    '.yaml': 'application/yaml'
}

then if someone makes a request for /blap with an Accept: application/blap+yaml, it should try /blap.byaml, and if that fails it should try /blap.yaml.

[fluffy-critter]

This could probably be implemented by treating composite types in the Accept header as dual types, e.g.

Accept: application/blap+yaml;q=0.7

could internally expand out to being equivalent to:

Accept: application/blap+yaml;q=0.7, application/yaml;q=0.69999

The way that Q factors are currently implemented is also kind of janky so this is a good opportunity to address that.