This provides a better input experience via
simulate physical keyboard, it converts
SDL_KeyboardEvent to proper HID events and send it
via HID over AoAv2.

This is a rewriting and bugfix of the origin code
from [@amosbird](https://github.com/amosbird).

Make sdl_keymod_to_hid_modifiers() more readable

Support MOD keys in HID mode

Enable Ctrl+V on HID mode

Support to send media events from hid_keyboard

Use existing --serial to replace --usb

Use explict option for input mode

Move HID keyboard setup code into functions

Send HID events in separated thread

Let libusb handle max package size

Fix HID keyboard report desc

Update README.md