Add highlight groups for the signs and the lines.

This allows users to customize the coloring of Syntastic's signs, and
also provides them a way to highlight the entire line using :sign's
linehl feature.  It preserves the current default behavior of using
the error and todo highlight groups if the customized ones aren't
present.  If the linehl group doesn't exist, then it's the same as not
specifying one, so no special highlighting for the line will occur.