Improve dimming class functionnality

[regisphilibert]

There is currently a way to choose the dim level of images (although this is not documented).

But I find its UX not very intuitive as users have to be educated about tachyon color classes.

We should only expect an incremental number. I'm thinking 1 to 9 which will translate to bg-black-10 to bg-black-90. We should also support the former behaviour (while not documented it might be used still)

[regisphilibert]

Should be included in #467