[converter.js] Add support for quoted-printable

[Infocatcher]

Requested here: http://akelpad.sourceforge.net/forum/viewtopic.php?p=28783#28783 (and not only here…)

https://en.wikipedia.org/wiki/Quoted-printable
Example:

7 =D0=BF=D1=80=D0=B0=D0=B2=D0=B8=D0=BB =D1=81=D0=BE=D0=B7=D0=B4=
=D0=B0=D0=BD=D0=B8=D1=8F =D0=BA=D1=80=D0=B0=D1=81=D0=B8=D0=B2=D1=8B=D1=85 =
=D0=B8=D0=BD=D1=82=D0=B5=D1=80=D1=84=D0=B5=D0=B9=D1=81=D0=BE=D0=B2 / =D0=A5=
=D0=B0=D0=B1=D1=80=D0=B0=D1=85=D0=B0=D0=B1=D1=80

[Infocatcher]

Some implementation: https://github.com/mathiasbynens/quoted-printable/blob/master/quoted-printable.js