filter: add gravatar scripts

The lua one is hugely faster than the python one, but both are included
for comparison.

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

filters/email-gravatar.lua

@@ -0,0 +1,25 @@
+-- This script may be used with the email-filter or repo.email-filter settings in cgitrc.
+-- It adds gravatar icons to author names. It is designed to be used with the lua:
+-- prefix in filters. It is much faster than the corresponding python script.
+--
+-- Requirements:
+-- 	luacrypto >= 0.3
+-- 	<http://mkottman.github.io/luacrypto/>
+--
+
+require("crypto")
+
+function filter_open(email)
+	buffer = ""
+	md5 = crypto.digest("md5", email:sub(2, -2):lower())
+end
+
+function filter_close()
+	html("<img src='//www.gravatar.com/avatar/" .. md5 .. "?s=13&d=retro' style='height:10pt;width:10pt'> " .. buffer)
+end
+
+function filter_write(str)
+	buffer = buffer .. str
+end
+
+