Lexer: Complain if @> found but not in tag mode. Also, bare @ is invalid.
Added test_cliexec.py with some basic tests for cliexec. Fixes #24.

These tests aren't much, but at least cliexec won't completely break and go
unnoticed. The tests can be run through cliexec-tests in the Makefile.am, or
invoked directly through running test_cliexec.py.

I'm considering folding the failing tests into it too, because I've had issues
with failing tests failing differently in the past and going unnoticed. But,
for now, this just fixes #24.