add Content-Disposition and Filename to the payload of incoming webhooks

for each message part. The ContentDisposition value is the base value without
header key/value parameters. the Filename field is the likely filename of the
part. the different email clients encode filenames differently. there is a
standard mime mechanism from rfc 2231. and there is the q/b-word-encoding from
rfc 2047. instead of letting users of the webhook api deal with those
differences, we provide just the parsed filename.

for issue #258 by morki, thanks for reporting!

webapi/doc.go

@@ -347,6 +347,8 @@ Example JSON body for webhooks for incoming delivery of basic message:
 				"charset": "utf-8"
 			},
 			"ContentID": "",
+			"ContentDisposition": "",
+			"Filename": "",
 			"DecodedSize": 17,
 			"Parts": []
 		},