Set the header bits correctly.

The current header settings claim that coloured text is available, which
it is not. I'm adding that presently, though, so that's okay.
Implement @set_text_style

The VT100 supports underline, bold and reverse, but not more than 1 at
once.
Colour and style!

Supports foreground and background colours, and all text styles,
including combining them.
Fix mapping of accented characters.

Everything except the oe and OE ligatures is now rendering properly.
Not sure why those are not supported.