* Update colors.nim
Added `lightgray` alias to `lightgrey` and `...grey`aliases for the rest of the gray colors.
Added color `rebeccapurple`.
Fixed the incorrect values for the `PaleVioletRed` and `MediumPurple` colors.
This module should now be matching the CSS colors.
I used the seq[tuple] syntax for defining the names.
* Document colors changes.