add a formula for "redpen"

RedPen is a proofreading tool to help writers or programmers who write
technical documents or manuals that need to adhere to a writing
standard. RedPen automates the verifications of input documents written
in natural languages.
It seems ```lib exec.install bin, lib``` does not work well,
but ```libexec.install %w[bin lib]``` works.
So I kept this line.