Add equidistant lightness distributions support

[fedeci]

Description

This allows the user to input a lowest and highest ratio (with additional possible ratios inbetween), and then choose a number of additional swatches to generate that will "fill in" the gaps. This closes #77

Motivation

A user may need to specify more than two required ratios and this allows him/her to do so.

Screenshots

To-do list

• [x] I have read the CONTRIBUTING document.

• [x] This pull request is ready to merge.

[NateBaldwinDesign]

@fedeci This seems like a pretty useful addition. I can see a lot of value in quickly entering a small number of ratios and then being able to generate _n_ additional swatches with even distribution.

However, this PR replaces the existing distribution feature, which also has its usefulness. We should not replace the existing distribution feature with this one.

I think that this feature is more related to ratio addition than it is to distribution. To that, perhaps this feature should coincide with the previous distribution feature. Here is a visual of how we could incorporate this new feature with the existing (within the UI). Providing a subheading for this section as "Add distributed ratios" (with classname spectrum-Detail) will help setup context. Then labeling the input "Quantity" can be much simpler regarding what the input relates to. Finally, placing the button right next to the input with a label such as "Add between" will help to clarify to users that the new contrast ratios will be distributed between the lower and higher contrast ratios entered above.

[fedeci]

I am going to work on this!