Linker.call keyword arguments as parameters

This shouldn't change any existing functionality, but it does cause the local `params` in `Linker.__call__` to become an empty dictionary, by default, instead of `None`.

That aside, keyword arguments (other than `expand`) given to `Linker.__call__` will now be passed as query parameters, which is useful for calls like:

    hc.emoticons(type='group')
    hc.rooms(include_archived=True)

Note the use of an underscore instead of a dash; the conversion is done automatically.