makefile: only display lua message once

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
email-gravatar.lua: fix for lua 5.2
email-gravatar.py: fix UTF-8
email-gravatar: fix html syntax issues

* make ampersand a html entity
* add required alt attribute
* add required img end tag
tests/: Add t0111-filter.sh

This adds basic tests for all types of exec filters.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>