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