Enhancements: Custom Cell Rendering/Retention Charts

[gryevns]

Hey, couple of suggestions/enhancements - want to know your thoughts before raising any PRs...

1. Custom Cell Render

It would be ncie to render something other than   within cells, e.g. the value with unit.

<HeatMap render={value => `${value}%`} />

2. Background Opacity

Currently setting the background & opacity means any textual value (e.g. above) inherits the opacity - instead it would be nice to use rgba - this means converting from hex to rgba to maintain backwards compatibility.

3. Retention Chart Support

The current gradient implementation works on a min/max across the whole dataset, whereas, for a retention chart the first value of each row is the "base" and the subsequent row values have a gradient compared to this value.

<HeatMap type="retention" />

[arunghosh]

@gryevns The above 3 features looks good. I happy to have the pull request for these features. Thank you :)