Add a mode to exec code in %debug magic
Document the new usage of %debug
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
raise UsageError for unsupported GUI backends

gives clearer, more concise message without traceback

closes #3609
Merge pull request #3071 from tkf/drun

Extend the `%debug` magic to also run a statement under the control of a debugger, if called with arguments.

If called with no arguments after an error has occurred, it continues to work as before.
Merge pull request #3623 from minrk/pylabusageerror

raise UsageError for unsupported GUI backends
Merge pull request #3618 from minrk/ansilinux

expand terminal color support (16 and 256 ANSI colors)