[fixes #36] add a fallback function for wp-cli plugin list response

[Sean12mps]

Suggested solution for issues #36 : Maybe we could try to have a fallback function if the json_decode function returns a null? Data that we need are only the strings between [ and ]. Later on, if the substring is found, we can json_decode it.

[mustela]

Let's try this approach an see how it goes!