Add charset option in Content-Type

Most mail clients, where facing an HTML email without declared encoding,
try to detect it or assume UTF-8. However, the correct behavior is to
assume latin-1 encoding, which some mail clients do. With those clients,
non-ASCII characters were broken.

Explicitly stating that the encoding is UTF-8 fixes that.
Improve maildir-feed-rss detection

maildir-feed need to run maildir-feed-rss. It try to guess its location
by calling DIR/maildir-feed-rss, where DIR=dirname(argv[0])

That fails if maildir-feed is in $PATH: then dirname(argv[0]) is empty.
Detect this situation and call maildir-feed-rss in $PATH.

Fixes issue#5