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
Add filter method to HaproxyLogFile

This filter method is the one that will run all filters (defined
on filters.py module) passed to the command line argument -f.
Apply filters before processing commands
Merge branch 'filters'