Add $header_color_partial to allow partial coloring of headers.

Add $header_color_partial to allow partial coloring of headers.

When set, a regexp match will color only the matched text in the
header.  When unset (the default), the entire header will have color
applied.

With appropriate regexps, this allows coloring of just the header
field name.  Of course, it can also be used to highlight arbitrary
phrases in the headers too.