Improve handling encodings

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.

modules/joex/src/main/resources/reference.conf

@@ -231,7 +231,9 @@ docspell.joex {
           "-s",
           "A4",
           "--encoding",
-          "UTF-8",
+          "{{encoding}}",
+          "--load-error-handling", "ignore",
+          "--load-media-error-handling", "ignore",
           "-",
           "{{outfile}}"
         ]