Extract servers log from weechat IRC log

Summary:
Convert a weechat log using the specified line format into a JSON
servers log.

Line format: <date>\t<nick>\t<message>

Date format: YYYY-MM-DD HH:MM:SS

This script has allowed to get the 2016-2018 log entries, before
this service allows to add new entries.

Thanks to @Sandlayth to have provided such a log.

Test Plan:

$ egrep "\[.*\] .*" log | grep -v "nginx: [" | grep -v Wearg > candidates
$ $EDITOR candidates # clean the log, mainly to remove [] false positives
$ ./extract_from_weechat_log.py > log.json

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Sandlayth

Differential Revision: https://devcentral.nasqueron.org/D1350