FileEdit#prepend_line_if_missing

This method can be used to prepend environment variable before a script evaluation for example. 
We can still use a common #missing_line? method without having to pass a block can't we?

```
def append_line_if_missing(search, line_to_append)
  @lines << line_to_append if missing_line?(search)
end

def prepend_line_if_missing(search, line_to_prepend)
  @lines.unshift line_to_prepend if missing_line?(search) 
end
```