lib: add hook for preprocessing commands

lib: add hook for preprocessing commands

This patch adds a hook point intended to allow subscribers to modify the
raw text of a CLI command before it is passed to the rest of the CLI
pipeline. To give access to the raw text of the command, a new function
for executing CLI has been defined whose only difference from
`cmd_execute_command` is that it accepts the command to execute as a
string rather than as a string vector.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>