Add field type color_picker

[westonganger]

It would be super helpful to have color picker field type. I propose two different types of color pickers:

1. Custom color picker field where any color can be chosen from a typical color swatch by clicking on it.
2. Limited color picker where there is a limited set of colors that can be selected from using a select box. Maybe we could have a YAML field in one of the config files, where we can setup one or more different color groups for this which can be chosen per field eg. "color_group": "background_colors"

[westonganger]

I see that metafields already has a color field type. Why doesn't sections have this feature?

[westonganger]

2 - This can be resolved using the feature proposed here locomotivecms/steam#151

[westonganger]

Related to #1337