v0.2.0: CLI improvements

This release contains minor improvements to the errtrace code transformer
allowing it to fit more use cases.

Added

• cmd/errtrace: Add -l flag to print files that would be changed without changing them. You can use this to build a check to verify that your code is instrumented.

• cmd/errtrace: Support opt-out on lines with a //errtrace:skip comment. Optionally, a reason may be specified alongside the comment. The command will print a warning for any unused //errtrace:skip comments.

  if err != nil {
    return io.EOF //errtrace:skip(io.Reader expects io.EOF)
  }