Proposed Check: Flag Improper Use of string.find()

Title: Flag Improper Use of string.find()

Description The string.find() method is easy to mis-use. A common error is:

mystring='abc'
if mystring.find('def'):
  print("def is contained in {}".format(mystring)

The issue is that if string.find fails, it returns -1, which is "truthy". If string.find() locates the output at the beginning of the line, it returns 0 which is "false".

mystring='abc'
if mystring.find('a'):
  print("a is in {}".format(mystring))
else:
  print('a is not in {}'.format(mystring))

It would be awesome if flake8-bugbear could flag any logical test involving string.find() that does not compare to a numerical value.