Add support for a third color in face definition that controls
the underline and a 'c' attribute for curly underline (that takes
precedence over 'u' if both are specified)
Allow empty colors to mean default, so that `,,red+u` means the
same as `default,default,red+u`
Fixes#4138
Triple strings are now distinct from docstrings, triple strings
only preceeded by blanks on the line are considered docstrings.
Avoid highlighting of the closing marker using a lookahead, this
is not fully correct as it will break on a double quote triple
docstring containing a single quote triple string but that seems
improbable enough; if we encounter this in the wild we can split
the two docstring formats into separate regions.
