Use the logging module to control verboseness of program.
Remove commented out code from the program.
Reduce slightly the verboseness of aria2.
Move class definition before all functions.
Follow PEP-8 and add more spaces between functions.
Declutter a tiny bit by removing unnecessary parentheses.
Break (some) long lines.

There is more work to do, but that's for later.
Remove comment that local page option is for debugging.

It is also useful to download videos that are listed in preview pages, for
example.
Add some blanks for legibility.

We also break one if in two lines for happiness of static analysis tools.
Add some docstrings to some functions.

We may, potentially, use these to create doctests in the future.
Add spaces to make pep8 happy.