Support 1, 2, and 4 bit palette modes

**Is your feature request related to a problem? Please describe.**
Attempting to decode a PNG with a 4bpp palette results in a `ColorMode(Palette, 4)` error.

**Describe the solution you'd like**
I'd like it to "just work", perhaps providing the {1, 2, 4} bits in the 8 bit channel as before. (different types could also work, but would be more frustrating to deal with).

**Describe alternatives you've considered**
Converting the image to 8bpp ahead of time. (`convert 4bpp.png PNG8:8bpp.png` works for this)
Using the chunked api instead.
Using a different library.

**Additional context**
[test_image.png](https://user-images.githubusercontent.com/7027737/128823438-b0af5497-afec-4710-9472-920df994fdbb.png)