feat: configurable price gradients

[brunofjesus]

This PR adds support for user-configurable price gradients for both light and dark modes, it also includes default gradients for both light and dark modes.

[abhisheksoni27]

Hey, @brunofjesus saw your comment on #321 and got to this PR. Is this ready?

[brunofjesus]

@abhisheksoni27 I've been using it for a while and it seems good so far. Just updated the README and set the status of the PR to Open.

[achannarasappa]

Thank you for the contribution! The ability to customize price color makes sense for users with a light color terminal. I have a few suggestions on the overall solution:

1. Since a user will have their terminal color as either dark or light, both don't need to be specified in the config. I would suggest flattening to something like :

  colors:
    price-positive-start: "#22C55E"
    price-positive-end: "#15803D"
    price-negative-start: "#EF4444"
    price-negative-end: "#B91C1C"

2. Terminal background color is not auto-detected for other parts of the UI. If auto detection were to be added, light color terminal colors should be added for all elements including the ANSI fallbacks. This is a broader change and perhaps further from the intent of this PR.

[brunofjesus]

Thank you for the contribution! The ability to customize price color makes sense for users with a light color terminal. I have a few suggestions on the overall solution:

1. Since a user will have their terminal color as either dark or light, both don't need to be specified in the config. I would suggest flattening to something like :

  colors:
    price-positive-start: "#22C55E"
    price-positive-end: "#15803D"
    price-negative-start: "#EF4444"
    price-negative-end: "#B91C1C"

2. Terminal background color is not auto-detected for other parts of the UI. If auto detection were to be added, light color terminal colors should be added for all elements including the ANSI fallbacks. This is a broader change and perhaps further from the intent of this PR.

My system has both a light and a dark theme, I often switch between them depending on the time of the day. That's why I placed colors for both themes. Having both colorschemes on the config file was the cleanest way I could think of doing it.

I can extend this to other elements either in this PR or I can do it on another one, I'm fine with either approach.