Add filters

Filters are a way to trim down the amount of log lines that are
going to be passed to commands for further investigation.

Filters are meant to be used as a way to focus the commands output
to certain specific log lines, i.e. only run commands on log lines
with a specific IP, on SSL connections or that contains a path...
Add a --list-filters command line argument
Add --filter argument parsing logic

It works much like --commands but with an added bonus of being able
to pass arguments to the filters, so that you can actually filter
for a specific IP for example.
Document haproxy.filters module