Check if the strings 'for' and 'while' in function

[koddsson]

Fixes #119 by checking the given function for the strings "while" or "for". It has the obvious drawback of failing if the mere string is present.

Not sure if this is the best solution but I thought at least it would get a dialog started on how to fix this issue.

[timoxley]

Ideally we'd parse their solution into an AST and check for loop constructs that way… but this perhaps solves it the most simple way.