Add rimble-icons to rimble-ui repo

[x] Icons should accept a size prop
[x] Icons should accept a color prop
[x] Should have a default color for crypto icons (whatever default color is)
[x] Should have a 'inherit' as color prop value for all non-crypto icons
[x] Should be used with rimble-ui Icon component api
[x] Should be importable as named imports
[x] Should be importable as wildcard import { * }
[x] Should honor space properties from styled-system as rimble-ui Icon
[x] Should honor space properties from styled-system when imported directly via named import (when imported directly)
[x] Should be documented on Storefront (https://github.com/ConsenSys/rimble-storefront/pull/217)
[x] Rename all references of "crypto" to "tokens"
[x] Should honor theme color value from styled-system

ConsenSysMesh / rimble-ui