add support for creating data: uris

[wbolster]

something like this:

def build(*, data, media_type=None, format=None):
    ... # todo base64/urlencode based on format spec, default to autodetect,
        # e.g. based on len(data) or presence of non-ascii chars.
    return "data:{},{}".format(...)

[patarapolw]

Must be this code https://gist.github.com/jsocol/1089733

I wish this can be merged to data_uri package