Make it easier/possible to override baked in constants.

[CarlFK]

Make it easier/possible to override baked in constants.
a.
args.video_attribs,
example:

    elif args.video_source == 'ximage':
        # startx=0 starty=0 endx=1919 endy=1079 !
        video_src = """
            ximagesrc {attribs} name=videosrc
                   use-damage=false !
            """

--video-attribs "startx=0 starty=0" will be added to "use-damage=false" (good)
but there isn't a way to set use-damage=true (bad)

    elif args.video_source == 'ximage':
        # startx=0 starty=0 endx=1919 endy=1079 !
        video_src = """ ximagesrc {attribs} name=videosrc """
        video_attribs = "use-damage=false"
    ...
    # make a dict out of video_attribs and args.video_attribs, merge.

>>> d = dict([kv.split('=') for kv in "use-damage=false foo=bar".split()])
# {'use-damage': 'false', 'foo': 'bar'}
>>> d.update({"foo": 'baz'})
>>> d
{'use-damage': 'false', 'foo': 'baz'}
>>> ' '.join(["{}={}".format(k,d[k]) for k in d ] )
'use-damage=false foo=baz'

Maybe code the defaults as a dict, but accept a gst style string to keep it looking consistent with other gst pipelines.

Then the same thing for args.video_elements