Allow change username & ignore certain exceptions

[palvarezcordoba]

• You can add show_custom_username=True when instantiating Slackify to send messages with a custom username. By default uses hostname@task_name. It can be overrided passing a username kwarg to Slackify. In the context of startup/shutdown messages there is not any task name so we will use celery by default, it can be overriden passing service_name kwarg to Slackify.
• Now you can silent certain exceptions using the ignore decorator like this:

from . import app
from slackify import ignore
from blah.exceptions import Timeout
from abc.exceptions import AwesomeException

@app.task(name="some_task")
@ignore(Timeout, AwesomeException)
def some_task(): pass

closes #8 closes #10

[crflynn]

Hey @palvarezcordoba sorry for letting this fall through the cracks. I should have some time this weekend to go over this. Should I close the previous PR and consider this one instead?

[palvarezcordoba]

@crflynn I've closed it.
This is the good PR. Here I don't use tricks involving inspect.trace.
I haven't written the tests yet.

[crflynn]

Hey I think these are good features to include. I could help write the tests out if you'd like to collaborate on this.