expand ANSI color support

adds support for 256 colors

does not yet add support for themed 16-color mapping ([01;38;5;[0-15])

closes #3617
add ansi background colors
add indexed-color in 256-color support
be pedantic about VT100 vs ANSI in comments
add ANSI color test notebook to examples/tests
Merge pull request #3618 from minrk/ansilinux

expand terminal color support (16 and 256 ANSI colors)