filter: return on null filter from open and close

So that we don't have to include the if(filter) open_filter(filter)
block everywhere, we introduce the guard in the function itself. This
should simplify quite a bit of code.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
filter: add support for email filter

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
filter: add simple gravatar email filter

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
filter: lua error reporting

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