adds basic plugin review via wp-cli only

- defines a new type of review:  “VIP Plugin Review” which applies the
basic VIP checks so far available only to themes to plugins as well
- adds two new wp-cli subcommands to vip-scanner:  scan-plugin and
analyze-plugin
- adds and abstracts some functions in the vip-scanner base class to
handle plugin reviews
- A new PluginScanner class makes it possible to scan or review any of
the following:  installed plugins (either single-file or directory) or
a plugin file or directory that exists anywhere on the filesystem (even
if not installed)
- WordPressCodingStandards check is now aware of whether it can
shell_exec() or not, and warns and quits if it cannot.