Rune - Add File.format method

Rune - Add File.format method

* Add File.format, e.g. stdout->format("printflikectl", ...) and
  test against most common cases.

* Note that in Rune var-args functions can case types, so the printf-like
  control does not implement 'l' or 'z' or 'q' or things like that.
  It implements the '#', '0', '-', '+', and ' ' prefixes and it
  implements 'd', 'u', 'o', 'x', 'f', 'g', 'e', 'p', 'c', and 's' controls,
  plus appropriate upper-case controls and field width / precision
  specifications.  '*' is also implemented for field width / precision.

  Unlike C, integer arguments can be of any signed or unsigned integer
  type, floating arguments can be any of the floating types, and '*'
  field widths can also be any signed or unsigned integer type.

* Flesh out the printing of floating point numbers.