Little bit of linting + two new features.

When using this on a project I'm working on it was copying a class called `center` and applying it to all my `td`s. So I decided to make an option to turn on or off that feature. 

* Added the ability to pull from a data `attr` if a `th` is blank
* Added the ability to turn off class copying from `th`s
* Linted code a little.