[feature] add user defined backlog command via txt files

[Gifford47]

it would be super handy if you could send custom commands in the "send config" window. you could take a new file-input field where you select a txt file that contains the commands in each line. this is then read in and sent to the esp when you click "ok".

this way you have a different txt file for each module, for example.
the backlog command already exists in "https://github.com/tasmota/tasmotizer/blob/release/tasmotizer.py#L243".

it could look something like this...