FileEdit#prepend_line_if_missing

This method can be used to prepend environment variable before a script evaluation for example. 
Hi @lamont-granquist, thanks for reviewing.

I believe we would lose what we gained from the refactorisation and be back at the starting point. We want to outsource the common behavior "If found return 1 and allow a block of code to be run", if not I do nothing and return 0". That is maybe just the name of the method which is not correct.