"after some quick testing, the execution time got halved (32-ish
seconds to 16-ish seconds) when going through all journal entries."

Closes #63.

Add log namespace support which is added since systemd v245

This change enables to add extra fields to JournalHandler in a
configuration file loaded by `logging.config.fileConfig`, which only allows positional
parameters:

class=systemd.journal.JournalHandler
args={'level': INFO, 'SYSLOG_IDENTIFIER': 'my-cool-app'}

[zj: originally the patch added a new positional parameter to
__init__(), but that is not backwards compatible. So I added a new
classmethod to allow the positional parameters to be passed.]

When running the tests in Fedora's mock, the test would
fail because NOTIFY_SOCKET is set to /run/systemd/nspawn/notify, and
we get a permission error.

`strftime("%s")` is not in the official python documentation but in my system (ubuntu 18.04 python 3.6.9) it is not aware of the object timezone and will return the wrong value if the timezone is specified and is not the system local one.

There are multiple ways to ensure a python `datetime.datetime` is in local timezone, the easiest (with python 3.3+) is to call `.astimezone()` If one wants to support earlier versions of python an extra dependency might be needed like `dateutil.tz.tzlocal()`.