FileEdit#prepend_line_if_missing

This method can be used to prepend environment variable before a script evaluation for example. 
This should probably be:

```ruby
def when_missing_line(line, &block)
  if missing_line?(line)
    block.call
  else
    0
  end
end
```

So we make the block responsible for returning how many items were edited (right now its not a problem, but what if you use this somewhere else and the block makes two edits -- you have knowledge that the block only makes a single edit leaking into this method that wraps the block).