Added custom length to random letter and number commands

[jsilvermist]

Fixes #25

Used method from int and float ranges to add custom length for random_letter as random_letter_window and random_letter_and_number as random_letter_and_number_window.

Command name change may break custom user-created key bindings, not sure if this is an issue or not though, as these commands aren't in the documentation anyways.

[certainlyakey]

Would be very handy to have this functionality.