Stop using eval() for --output

[petdance]

Bill Ricker rightly suggests that --output should be done with some sort of string eval that we do, rather than arbitrary code execution. I want to limit the output to only special variables like $1, $2, etc.

This will slow down output, but that's not the bottleneck in an ack run anyway.

[petdance]

This has been implemented in ack3. We will not be backporting it into ack 2.