Update index.js

Added alternative search conditions

  "key={in}a,b"
  "At least one of these is in array"
  
  "key={nin}a,b"
  "Any of these values is not in array"
  
  "key={all}a,b"
  "All of these contains in array"
  
  "key={empty}-"
  "Field is empty or not exists"
  
  "key={mod}a,b"
  "Docs where key mod a is b"
  
  "key={gt}a"
  "Docs key is greater than a"
  
  "key={lt}a"
  "Docs key is lower than a"