Improve vimrc for prose-like files

vimrc: Improve handling of prose-like text files (markdown, taskpaper,
text, etc.)

- UTF-8 should be the default where possible.
- Use a more discrete paragraph mark in listchars.
- Add extra digraphs for em- and en-dash.
- Add commands to configure simple soft and hard wrap modes.
- Use soft wrap for all the prose-like text files.
- Add vim-airline and configure to taste.