Column option for 'focusable' needs a default value. Set to true, as that's the default behavior assumed by 'canCellBeActive'
No longer need to check if columns[cell].focusable is a boolean: since every column now is boolean by default, we simply return focusable at the end of the function. (The original 'return true;' is not needed because the default value here IS true.)
Merge pull request #538 from dariusk/master

Adding default focusable column option, plus minor optimization to canCellBeActive