New filter: frontend

Filters log lines by the HAProxy frontend the connection arrived
from.
New filter: server

Filters log lines by the downstream server that handled the
connection.
Improve documentation

Create a new doc file for filters and add some headers on modules file.
Typo
New filter: response_size

Filters log lines by the response size (in bytes). Specially useful
when looking for big file downloads.
Fix tests logic

Some tests had assertTrue while it should be assertEqual and move
as much as possible out of the assertRaises contextmanager.
Add -n argument

This command line switch allows to negate the filters provided with
-f.
Update README
Remove unused import
Allow HaproxyLogFile.filter to reverse its output

This allows to be able to run filters like "all non SSL traffic",
or "all traffic but this IP".
Merge branch 'negate-filter'
flake8 fixes
PEP8 fixes thanks to http://pep8.me
New command: print

Prints the raw lines. This can be useful to trim down a file (with ``-s``
and ``-d`` for example) so that later runs are faster.
Fix messages about -l that now is --list-commands
Sometimes request path contains backslashes
Allow curly brackets on path
Use the reverse named argument on filters

This was forgotten, tiny but silly detail, when merging the negate
branch.
Add an idea on how to speed up the tool usage
Even diacritics are found on paths