adding new 'ensure' filter

Hi,

i was wondering if you would be interested in this addition? i use it to 'ensure' a minimum length of an array.

My use case was that i have a form that repeats a section, and based on another field i may need 3 or 6 or whatever of these sections filled in.

I've tried as best i can to keep it in line with your existing filters and I've added tests as well. (ran locally grunt travis and got all pass).

this is one of my first un-solicited contributions so please feel free to send back any changes you would require and ill try to get them done.