Add tint-shade color stack

[VinSpee]

Allow users to create a stack that includes both shaded and tinted versions of
a color at the same time.

Not sure how you want to approach things like the conjoin method that @scottkellum helped with – there didn't seem to be a global place to put it.

sample output:

[Snugug]

Thanks for the PR! I have some comments I'd like addressed before pulling this in.

1. Please refer to the Contributing Guidelines. Specifically, all new features need to include documentation and any argument with a defaulted argument needs a globally namespaced variable to be able to control it from outside of the mixin/function
2. The conjoin function is not a color function, and therefore does not belong in the colours partial.
3. You are hard-coding list positions into the stack even though the number of tints and shades that could exist could be more or less than 6
4. I'd like to see the list start dark and end light by default so popping off colors happens in the same way as tint-stack and shade-stack. This will mean that the chosen color will be in the middle of the stack
5. Because of the nature of this stack, I'd like to see a different default amounts stack used, with negative percentages for shade and positive percentages for tint.

For 4 and 5, take a look at this quick SassMeister I drew up to kinda explain what I mean.

[VinSpee]

Thanks, I'll dig into this this weekend.