Hex commands support

Hi Takasu,

Firstly let me thank you from Russia for the great open source tool :)
(sorry for my English, just in case)

Recently I started modifying the code to use it with some not supported GNSS receiver.
I hope that some things, that I needed and added before parse its binary protocol, will be useful for others.

I don't have any experience of contribution to opensource projects in GitHab, so if you have time and patience please tell me frankly if I do something wrong.
Is it convenient if I do pull requests step by step (one feature for request)?

First feature I want to suggest is a possibility to send start and stop hex commands, because in last version of rtklib 2.4.2 p7 the only ways to send commands with not ASCII symbols are
1) special packages with special headers for certain receivers (NVS, for example)
2) load binary files (but still can send whole command if there is a null terminated zero byte 0x0 in it).

So relying on your code, I add just a little modification, so now I can send hex command using !HEX option in cmddlg fields.
Example: !HEX 00 01 ab BC ff

Sorry for big text :) and thanks for attention