Html and text files are not fixed to be UTF-8. The encoding is now
detected, which may not work for all files. Default/fallback will be
utf-8.
There is still a problem with mails that contain html parts not in
utf8 encoding. The mail text is always returned as a string and the
original encoding is lost. Then the html is stored using utf-8 bytes,
but wkhtmltopdf reads it using latin1. It seems that the `--encoding`
setting doesn't override encoding provided by the document.
Each attachment is now first extracted into potentially multiple ones,
if it is recognized as an archive. This is the first step in
processing. The original archive file is also stored and the resulting
attachments are associated to their original archive.
First support is implemented for zip files.
