[Avatar] requires more customization

[uboness]

I was trying to use Avatar in its current form, but it's not working for me. Here're a few suggestions:

• Make color to be configurable property - usually the color of the avatar is associated with the user in more places, so mostly the color will need to be provided rather than auto-generated internally in the component

• More on color - currently the color in the Avatar is limited to a set of the available visualization colors. It would be nice to remove this limitation, and instead have EUI provide a set of color generators. These generators can:

1. generate a random color that fits Kibana's color scheme (some sort of pastel color generator)
2. generate a color (again, one that fits Kibana's color scheme) from a given seed (can either be a number or a string). This will bring quite a bit of powerful flexibility for creating consistent behavior. I think the best way would be to use HSL colors for that (I think it'll give use more control of the generated color scheme)... oh yea... and these functions should also accepts an alpha/opacity value (so hsla).

• Make the text fallback (initials) customizable - I'd expect the component to accept an "initials" prop. these should be max 2 characters (if longer string is provided, the component will only pick the first two).

[snide]

Makes sense. Kinda went in this direction with the badge component. Can do something similar.

[cchaos]

Also need the ability to specify circle (user) vs square (spaces) shape.

[cchaos]

It might be nice to be able to hook in to the Gravatar API to use images from that service if they've signed up for it.

[snide]

Closing this one. The above PR tackled most of the issues here.