Change default palette to be more general purpose

Before this commit, the default palette was based off of xterm-256color.
While that worked great for consumers that were building web apps that
targeted CLIs, it wasn't really a great default.

For those (like me) who would still prefer to use the xterm palette, you
can now load it from this other package:
https://github.com/bilalq/xterm-256color-palette
Rename "colorMap" to "palette"

This introduces breaking changes in the API
Update screenshot to reflect new default palette
Add section about custom palettes
Bump major version