Allow configuring specific headers in theme

[jaspervdj]

In addition to header, individual headers can now also be customized. The
configuration blocks under headers accepts a style list, a prefix string
and an underline string that is repeated match the width of the header.

For example:

patat:
  theme:
    headers:
      h3:
        style: [vividRed]
        prefix: '### '
        underline: '-~-~'

See #81