Initial support for custom bindings

In addition to supporting UI customizations, we plan to offer a basic way to create custom bindings: ```typescript export default { bindings: [ { key: 'C-c', command: 'new-window', }, { key: 'h', command: 'select-pane', options: ['-L'] } } ``` ```sh # tmux.conf bind C-c new-window -L bind h select-pane -L ```