Enable autogeneration of command-line tool documentation

Enable autogeneration of command-line tool documentation from the help output.  
In order to achieve this, we move from having a "bin" directory to using the "entry_points" mechanism.

Since Github is bad at displaying the diff, effectively the changes are:

- Move everything in `bin/` to `pwnlib/commandline/`
- Add `argparse` to the command-line scripts that didn't have it.
- Add links in the documentation
- Change from using setuptools `scripts` directive to using the `entry_points` directive