defgroup customization

Added `defgroup` and `defcustom` including some completions, insert lists (for the custom folder list), and proper keystroke binding.  Didn't include customizations for the vars discussed in the [issue 6](https://github.com/agpchil/mu4e-maildirs-extension/issues/6).

Moved the `defface` group to match extension name (it had `maildirs` as singular).

Changed `README.md` to suggest using customize and not `setq`.