Add more format support like *args

[betterlch]

like this:

def gettext(string, *args, **variables):
    """
        gettext(u'Hello World!')
        gettext(u'Hello %(name)s!', name='World')
        gettext(u'Hello %s!', 'World')
    """
    t = get_translations()
    if t is None:
        return string if not variables else string % variables
    s = t.ugettext(string)
    if args:
        return s % args
    else:
        return s if not variables else s % variables

[betterlch]

Also，add "{}".format(*args) support in the same way, it's easy to write the code and tests.

[betterlch]

like #170 maybe add a new func name fgettext to support format is better than changing func gettext