Adding default focusable column option, plus minor optimization to canCellBeActive

I was in the middle of updating the [Column Options documentation](https://github.com/mleibman/SlickGrid/wiki/Column-Options), and while researching `focusable` I noticed that it didn't get assigned a default value. In 5ad8c99 I've added the appropriate default (`true`) and in 9431ed9 I slightly optimized `canCellBeActive` now that we can assume a boolean.