edits to add discrete or continuous

[SwampThingPaul]

Hi Katie, Love your package!! I just added an ability to make the color palette discrete (like your base package) or continuous. Thanks again. You don't have to "accept" the changes if you don't want to.

[katiejolly]

Thanks for submitting a pull request! I'm not going to merge quite yet. A few thoughts:

• for the hex sticker, the package isn't actually affiliated with NPS so I don't want to use their logo
• for the continuous scales, my only hesitation is that I want to be able to sort colors before making them continuous, so maybe I'll look into that a bit