Summary of changes:
* Added duration to postcommand hook
* Switched main search operation to use `subproccess.run()` rather than running as an xonsh shell command - this a) allows us to capture stderr without needing a temporary file and b) avoids a weird broken-buffer state that results from running a fullscreen TUI and then programmatically editing the buffer
* Added support for immediately executing chosen command via `__atuin_accept__:` (like bash/zsh/fish)

Xonsh doesn't import private functions into the local namespace when sourcing a file