update and add Markdown Preview
MarkdownPreview: fix preview of Windows 1252 encoded files

Previewing Windows 1252 (cp1252) encoded files failed with:

"UnicodeEncodeError: 'charmap' codec can't encode characters in position
316-319: character maps to <undefined>".

The reason is 'markdown.css', it contains non cp1252 characters.
Fixed this by always generating utf-8 encoded output.