Replace config.yaml with environment variable

[nsbruce]

Depending on where one chooses to keep the tbmk executable, having a plain config.yaml file in the same directory is not ideal. For example, I have a ~/.local/bin folder where I put executables like tbmk. If more than one executable requires a config.yaml file this structure doesn't work.

One alternative solution is to set an environment variable, something like TBMK_DATA_DIR. It could also be added directly to the keybindings for each shell.

If you support this idea I'm happy to take a crack at it and open a pull request.