Add contentPath attribute

This adds a contentPath attribute that you can provide instead of a `getCellContent` method. I found that a lot of the time I just wanted a named attribute from a row rather than any formatting, and this allows that less verbosely. As it is a path, it's possible to do multilevel access e.g. `contentPath: 'foo.bar.baz`.

Of course, there is still the option to provide the function to do anything more advanced e.g. formatting etc.

(I am working towards #31 but want to do a bit at a time with some minor api changes first, as it will change how I need to refactor those features)