Allow custom user key-mappings

[mikeboiko]

I would love to remap send from Alt+Enter to Ctrl+Enter. Would you consider exposing the key mappings in telegramtui.ini, please?
I love the program by the way!

[vtr0n]

Hey!
I tried to add Alt+enter mapping, but I have a problem with npyscreen(a library which works with tui). Maybe I add a patch later.

[mikeboiko]

Sounds good, thanks!

[n0v1c3]

Second it as a tmux vim user. I love my key mapping.