allow to define a default mime-type to use when lookup in the internal table fails

instead of always returning 'text/plain'

To allow setting a different default mime-type the mg_get_mime_type() api has been
extended to require an mg_server parameter.

Note that using a static global as default mime type wouldn't have allowed us to
control it via a configuration option because there is not global initialization phase
and configuration options are provided (and specific) to each server instance.
This means that different servers can be configured to use a different default mime-type
(which is also a desired behaviour in many scenarios) when lookup fails.