Add ability for data_color to color cells row-wise instead of column-wise

[ryantaylor406]

Proposal

Currently, the data_color function can only colorize data based on column values. On the R GT package, they have a direction argument:

direction
Color computation direction
singl-kw:[column|row] // default: "column"
Should the color computations be performed column-wise or row-wise? By default this is set with the "column" keyword and colors will be applied down columns. The alternative option with the "row" keyword ensures that the color mapping works across rows.

It would be very useful to have this functionality to color cells based on row values.