Changes to allow color index with JXLS

The current JXLS code only allows color_name_t’s to be used for
defining color.  These few changes allow JXLS to use color index’s
everywhere color_name_t’s are used EXCEPT for in JXLSRange.  Currently,
the underlying xlslib does NOT include support for a color index in its
interface for “cellColor”.  If that is added, than a cellColorIndex
method could also be added.
Merge pull request #12 from redshoulderedhawk/master

Changes to allow color index with JXLS. This should fix #11.