Add support to an user defined progress bar inside download function.

[jcfaracco]

This commit just introduces a new argument to the download function: bar. The argument let the person who is developing code define his own progress bar using whatever he wants to. It also requires the pattern of passing the current size of the file and the total size.

An external function should be defined like

def my_progress_bar(cur, total):
    print(cur, total)

And then used as:

gdown.download(id=id, bar=my_progress_bar)

Signed-off-by: Julio Faracco jcfaracco@gmail.com