[FEATURE] gradient colors

**Is your feature request related to a problem? Please describe.**
only a static color for a foreground/background is allowed right now, which limits creativity

**Describe the solution you'd like**
being able to set the foreground/background color, to a horizontal/vertical/diagonal gradient.
ideally, we would have a Gradient trait with a `fn get_color(pos: XY, size: XY) -> Color` that would allow users to make their own gradients which would fit their needs

**Describe alternatives you've considered**
if having gradients isnt very important you can just use a static color, if you need them the only option is using the Canvas or writing your own View

**Additional context**
was already mentioned [here](https://github.com/gyscos/cursive/issues/607#issuecomment-940227726), seems like theres no progress on it tho
i could write the code for this, but im not sure what the api should look like
heres a [gist](https://gist.github.com/therealolie/a119859fbf377fde81174ad514c7913d) with a proof-of-concept