Give compatibility with Custom Mobs plugin

This would extend the compatibility of denizen with all the Custom Mobs plugins or mobs created by command blocks.
This would also solve this issue because MM offers commands to spawn and remove mobs, with the mobnames argument you can now check for the display name that you can set in the config (I think all the custom mobs plugins offer this feature) : https://github.com/Morphan1/Depenizen/issues/20

I used contains() instead of equalsIgnoreCase to check for the name because some plugins could add informations to the mob's name like the level or the health.

I tested this in local and should work fine.
I'm not sure of how to do pull request so i copied the new code from my local file in the file here, but everything should be in the right place :)
I don't think it would work. if for example i write: - listen ... target:zombie mobnames:OldZombie and I kill a Zombie called NewZombie, the first foreach would set it to true (because I killed a zombie), the second foreach would be irrelevant because count_it is already true and then so it will count the entity even if the name is different. 
I could set it to false if no corresponding mobname is found but then it would overwrite the result of the target foreach.
I thought this was the safest way to avoid bugs, but I'm not an experienced programmer so probably I'm wrong.