Add bundle size increase

[maoueh]

A nice thing that would help us develop fast loading website is to compute the amount of kilobytes a particular PR add to the final bundle size.

How exactly all this would be done, it's unclear. However, probably a previous build size would be needed (or some fork task that clone the original branch, build it and store the bundle size for future comparison inside the same build).

A research of available tools would be a good start to see how we could implement the feature. Than, danger-samsao would report back when there is an increased > threshold (define in the config file).