recognize more charsets than utf-8/iso-8859-1/us-ascii when parsing message headers with address

as they occur in From/To headers, for example: "From:
=?iso-8859-2?Q?Krist=FDna?= <k@example.com>".  we are using net/mail to parse
such headers. most address-parsing functions in that package will only decode
charsets utf-8, iso-8859-1 and us-ascii. we have to be careful to always use
net/mail.AddressParser with a WordDecoder that understands more that the
basics.

for issue #204 by morki, thanks for reporting!

message/addr.go

@@ -0,0 +1,29 @@
+package message
+
+import (
+	"fmt"
+	"net/mail"
+
+	"github.com/mjl-/mox/smtp"
+)
+
+// ParseAddressList parses a string as an address list header value
+// (potentially multiple addresses, comma-separated, with optional display
+// name).
+func ParseAddressList(s string) ([]Address, error) {
+	parser := mail.AddressParser{WordDecoder: &wordDecoder}
+	addrs, err := parser.ParseList(s)
+	if err != nil {
+		return nil, fmt.Errorf("parsing address list: %v", err)
+	}
+	r := make([]Address, len(addrs))
+	for i, a := range addrs {
+		addr, err := smtp.ParseNetMailAddress(a.Address)
+		if err != nil {
+			return nil, fmt.Errorf("parsing adjusted address %q: %v", a.Address, err)
+		}
+		r[i] = Address{a.Name, addr.Localpart.String(), addr.Domain.ASCII}
+
+	}
+	return r, nil
+}