Simple mime decode filter using mime.Decoder go libs

Because reading utf8, and other encoded subjects in mail headers is no intuitive, this filter translate mime encoded text.

utf8submimedecode was needed for another project subayes, a mail subject ham/spam discriminator.

Bonus ! golang mime decoder also decode iso-8859-1

Exemple

$ echo "Re: =?UTF-8?B?w4l0YXQgQ0VU?=" | utf8submimedecode
Re: État CET

$ echo '=?iso-8859-1?q?this=20is=20some=20text?=' | utf8submimedecode
this is some text

$ echo '=?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?=' | utf8submimedecode                       >
If you can read this yo