Added-

columns option:
Specify number of columns to spread options across. will scroll vertically
if needed

column_width option:
Specify width of columns to spread options across. Popup will retain
240px height, but spread options across columns with given width,
scrolling horizontally if needed. Must be assigned as string (ie: "120px")
Assign "fit" to make the columns as wide as the widest text item in
the select. This will prevent ellipsis from appearing for items cut off.
Changed checkboxes to add and remove tags on select
Merge pull request #2 from FesterCluck/checkbox

Adding checkbox columns, bug fixes to allow check boxes to modify tags in list