Option to mention someone

[dahlia]

Thank you for making a useful library!

This patch adds a new option named mention which purposes to send messages with mentioning someone. If the option is omitted it behaves as like it has done.

An example code follows:

import logging
from slack_logger import SlackHandler, SlackFormatter

sh = SlackHandler(
    'YOUR_WEB_HOOK_URL',
    username='python-slack-logger',
    icon_emoji=':thinking_face:',
    mention='U012ABC34'
)
sh.setFormatter(SlackFormatter())
logging.basicConfig(handlers=[sh])
logging.warning("Trying SlackHandler's mention= option...")

A message the above code sent looks like:

Postscript: Although its setup.py indicates the project is licensed under MIT, it would be more clear if the repository contains an explicit LICENSE file as well. GitHub also can recognize the LICENSE file so that it shows project's license on the overview bar.

[junhwi]

@dahlia Thank you for adding great feature!