security-checker gadget

add sensiolabs security checker.

https://security.sensiolabs.org/

i added a new database column `description` in the issue table. 
i think it is a good idea to allow markdown, isn't it?

waiting for #65 because of migrations.
Not just specific to this gadget, but: why do we have default values for the bin option if we only use the gadget  with the binaries from the composer dependencies. I would suggest having only default values, when there is at least one use case for  (tests do not count as a use case :smile: ).