Add data visualization examples using Altair

[JWCook]

Based on forum conversation here: https://forum.inaturalist.org/t/what-would-you-like-to-learn-about-getting-data-from-the-system/19225/14

And blog post here: https://www.inaturalist.org/journal/muir/32944-alaska-inat-passes-100k-observations-5k-unique-taxa

Other misc changes:

• Update changelog and list of implemented endpoints
• Add some more dependencies to Dockerfile
• Add 0 and 0.0 to valid 'falsy' values to not strip from request params
• Fix bug with processing datetime parameters
• Add function to help with getting date ranges for histograms with custom metrics