Added support for different file encodings

The title says it all.

- Added iconv-lite
- New ini setting editor.encoding
- Failback to utf-8
- UI Header shows active encoding
This check should be moved after https://github.com/slap-editor/slap/blob/master/lib/ui/Editor.js#L23 to look something like `if (!iconv.encodingExists(opts.encoding)) opts.encoding = 'utf8';`.