[Fix #888] Make Style/InverseMethods aware of exists?

Fixes #888.

This PR makes `Style/InverseMethods` aware of `exists?`.
Probably simpler than implementing a new named cop. And there will be
no overlap with existing cop as reported in #941.

Author: koic

config/default.yml

@@ -1150,6 +1150,10 @@ Style/FormatStringToken:
   AllowedMethods:
     - redirect
 
+Style/InverseMethods:
+  InverseMethods:
+    :exists?: :none?
+
 Style/SymbolProc:
   AllowedMethods:
     - define_method