Request: Define Row Groups Based on an Existing Column

Hey there, this package looks promising. It would be nice to have an automatic definition of how row groups should be defined based on the values of a column, instead of: 

```
tableHTML(mtcars, 
          row_groups = list(c(10, 10, 12), c('Group 1', 'Group 2', 'Group 3')),
```

I'd like to write:

```
tableHTML(mtcars, 
          row_groups = "cyl"),
```

