CLI command app:check-code has been extended

[nickvergessen]

It now not only allows to check for the use of private vs. public api, but also for deprecated methods, for validity of the info.xml, all checks being enabled by default.

Usage:
 app:check-code [-c|--checker="..."] [--skip-validate-info] app-id

Arguments:
 app-id                check the specified app

Options:
 --checker (-c)        enable the specified checker(s) (default: ["private","deprecation","strong-comparison"]) (multiple values allowed)
 --skip-validate-info  skips the info.xml/version check

However this only has a short section in the admin doc (line 141-151) and I think that should be okay to be small/short.

More docs should be in the developer section.

[carlaschroder]

#1771 please review