Adds `lf hid pack` and `lf hid unpack` offline commands. (#614)

* These are used to pack and unpack the bit length, facility code and card number from "short" HID Prox IDs.
* This also simplifies some repeated code that is used to parse hex digits in lfhid, and adds basic client-side validation to the `lf hid clone` and `lf hid sim` commands, for when an ID that is too long is entered.
* Implements parity checks/calculation for 26-bit H10301 cards.
* Adds links to useful resources for HID Prox cards.
* Adds an explicit warning about the lack of parities on non-26-bit cards.
* Changes all the examples to use 26-bit IDs that have a parity bit set.