This commit creates the mime/quotedprintable package. It moves and
exports the QP reader of mime/internal/quotedprintable.

The code is almost unchanged to preserve the commit history.

Updates #4943

Change-Id: I4b7b5a2a40a4c84346d42e4cdd2c11a91b28f9e3
Reviewed-on: https://go-review.googlesource.com/5940
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>