filter: add page source to email filter

Since the email filter is called from lots of places, the script might
benefit from knowing the origin. That way it can modify its contents
and/or size depending.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

filters/email-gravatar.lua

@@ -9,7 +9,7 @@
 
 require("crypto")
 
-function filter_open(email)
+function filter_open(email, page)
 	buffer = ""
 	md5 = crypto.digest("md5", email:sub(2, -2):lower())
 end