Add new linkcode_link_text config value which can be either a string or a
callable used to generate the text for generated links. If value is callable, it
will be called with same arguments as linkcode_resolve to get the text for
each link. Otherwise it will be used directly. Either way, the result is passed
to str to get the text for the link. Default is '[source]', so it is
completely backwards compatible with previous version of the extension.