encoding: add lenientQPReader to handle malformed quoted-printable encoding

[dejanstrbac]

Go's strict mime/quotedprintable decoder rejects slightly malformed QP encoding (= sequences).
RFC 2045 says implementations should be lenient. We encode the '=' as '=3D' so the standard QP decoder will decode it as a literal '=' character.

6.7. Quoted-Printable Content-Transfer-Encoding, Page 21, section 2:

      An "=" followed by a character that is neither a
      hexadecimal digit (including "abcdef") nor the CR
      character of a CRLF pair is illegal.  This case can be
      the result of US-ASCII text having been included in a
      quoted-printable part of a message without itself
      having been subjected to quoted-printable encoding.  A
      reasonable approach by a robust implementation might be
      to include the "=" character and the following
      character in the decoded data without any
      transformation and, if possible, indicate to the user
      that proper decoding was not possible at this point in
      the data.