shell: add meta-keys

Added following meta-keys:
Ctrl-B - moves the cursor backward one character
Ctrl-D - deletes the character under the cursor
Ctrl-F - moves the cursor forward one character
Ctrl-K - deletes from the cursor to the end of the line
Alt-F  - moves the cursor forward one word
Alt-B  - moves the cursor backward one word

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>