Support custom palette in editor and resource files
**Is your feature request related to a problem? Please describe.**
We can run a pyxel game with a custom palette, but we can’t use it in the editor.

**Describe the solution you'd like**
Idea 1: add a command-line parameter to the editor like `--palette 1a1c2c,5d275d,b13e53,ef7d57`
This could be a little unconvenient to use though.

Idea 2: add a param like `--palette path/to/file` that reads the palette from a file.  Could use a text file format (lospec for example has multiple text formats for each palette) for easy parsing, or a PNG file with 1px per colour but that would require PIL or pypng to analyse.  I have a clone of the repo and added this option for my own usage, so I can send a PR.

Idea 3: update resource file format to store palette in the resource file.  Load automatically when opening a resource file with a custom palette.  To save a custom palette, either keep it a secret feature (people have to add palette file to the zip) or add a palette tool in editor.  I probably can’t make a PR for this, and you’re doing the rust rewrite so probably better to wait for that.

**Describe alternatives you've considered**
See above

**Additional context**
