FileEdit#prepend_line_if_missing

This method can be used to prepend environment variable before a script evaluation for example. 
Is refactoring this to remove duplicate code yet preserve the method signatures too complex? e.g.

```
def add_missing_line(search, line, prepend = false)
  count = 0

  unless @lines.find { |line| line.match(search) }
    count = 1
    if prepend
      @lines.unshift line
    else
      @lines << line
    end
  end

  def prepend_line_if_missing(search, line_to_prepend)
    add_missing_line(search, line_to_prepend, true)
  end

  def append_line_after(search, line_to_append)
    add_missing_line(search, line_to_prepend)
  end
```

Eh. Not having named method parameters probably makes that confusing.