Enable grouping of columns for show/hide

Per the developer's configuration, adds 'group buttons' for showing
and hiding multiple groups at one time.  The 'indeterminate' property
is used on the group button when not all columns within the grouping
are the same checked state.
Additional styling for ColVis grouping
Documentation for ColVis grouping functionality
Update docs with link to 'Grouping Columns'
Merge pull request #15 from dustinrc/master

 Enable grouping of columns for show/hide