Add keybind configuration

[dxrcy]

Keybinds (control) can be configured in ~/.config/tui-chan/keybinds.conf.
Default controls are the same.
Configuration file is parsed at the start of program execution, and file is created if not already existing, with default controls.

Added keybinds module, and updated main.rs and README.md.
Changes have been tested and linted.

Fixes #3

Reference:

• keybinds/key.rs for parsing single keybind definitions
• keybinds/mod.rs for definition of Keybinds struct, and parsing of file
• keybinds/file.rs for .conf file reading and writing

Possible issues:

• Config folder only works for Linux. Could be solved with dirs-next crate
• Only one keybind per action allowed currently: cannot use both WASD and arrow keys for instance

[tuqqu]

Thank you for this PR. The PR looks tidy and clean, great job!

A few comments:

1. Please, take a look at the Clippy error.
2. app.rs has a help menu to look up the keys, which should be updated accordingly. The simplest way to address this would be to pass keybinds to App::new as an argument to build the message.
3. Let's not fail upon not being able to find the config name folder, but rather log this error to stderr and continue execution.

[dxrcy]

All problems should be patched now :)

[tuqqu]

Thank you for the contribution!