Save/Restore Buffers Feature

Added support for saving and restoring buffers for panes. Preliminary support is for Bash only (like the recently added Bash history restore support). Also added optional support for ansi color codes in saved pane buffers. Both of these options are disabled by default but can be enabled by adding the following options to the `.tmux.conf` file:

```sh
    set -g @resurrect-save-pane-buffers 'on'
```

```sh
    set -g @resurrect-enable-ansi-buffers 'on'
```

This feature is meant to be used in conjunction with the Bash history save feature but is not dependent upon it. The README has been updated as well.