Remove color and resolution limits
**Is your feature request related to a problem? Please describe.**
Resolution and color palette size limits seem pretty arbitrary. (I understand that the reason for the resolution limit is a limitation imposed by the way the renderer was implemented, see #26 #22)

**Describe the solution you'd like**
I'd like to be able to set whatever resolution and whatever palette I want. A maximum resolution of 1024x1024 and palette length of 256, however, would be OK. The default palette could still be the Pico-8 palette.

**Additional context**
Due to the limitations of the current implementation, fairly large changes would need to be made to accommodate larger screen sizes, but would likely be worth it. By contrast, removal of the palette length appears to be relatively easy to implement. 

It's worth noting that many 8-bit systems, such as the NES or Commodore 64, had specs exceeding the current limitations of Pyxel. (NES had more colors, C64 had a higher resolution)