FileEdit#prepend_line_if_missing

This method can be used to prepend environment variable before a script evaluation for example. 
Based on comment (below) about when_missing_lines, this should be written as:

```ruby
 def prepend_line_if_missing(search, line_to_prepend)
   when_missing_line(search) do
    @lines.unshift line_to_prepend
    1
  end
end
```