shell: Add built-in shell commands

Added optional shell commands:
- clear - for clearing terminal
- history - commands history
- resize - terminal resize
- shell - controling echo and colors

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>