Workaround for outlook, who doesn't like quoted-printable encoding

For example, outlook 2013 plus would not interpert the following html
(encoded via quoted-printable encoding):
<a href=3D=22http://engineering=
.como.com/ghost/signup/MTQxNTcwNzM5MTYwN3x5dXJ5QG5peC5jby5pbHx3NVBmUUN4RGZr=
cGhKc3FPOElybXNZNWtzR0FMTU9tRFI5UlNyRWQ4SGJZPQ=3D=3D/=22 style=3D=22color:
=

The workournd forces node-mailer to user base64, which outlook understands
well.
Merge pull request #4349 from yurynix/master

Workaround for outlook, who doesn't like quoted-printable encoding