Added in feature to assign keyboard or gamepad with the -N or -R option

[DesiOtaku]

Also cleaned up the help text

Agreement

By creating a pull request in stk-code, you hereby agree to dual-license your contribution as
GNU General Public License version 3 or any later version and
Mozilla Public License version 2 or any later version.

This includes your previous contribution(s) under the same name of contributor.

Keep the above statement in the pull request comment for agreement.

Changes as per this bug: #5213

Please let me know if the style is too off or any other changes needed to be made. Thank you.

[Alayan-stk-2]

Thank you for your PR.

Don't forget to pay attention to the coding style for the if/else braces next time :)