shell: add an option for minimal default configuration

The default shell configuration has heavy flash and memory requirements,
requiring project maintainers to set many configuration options to "n"
to keep flash and memory requirements within reason.

This adds a new configuration option, CONFIG_SHELL_MINIMAL, which will
disable flash and memory heavy options by default, and allow project
maintainers to select/imply only the options they want.

On a quick test from an ARM board I'm working on, enabling this option
cut flash space requirements by ~8 KB, and memory requirements by ~1 KB.

Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>