Add alert handler to create Datadog Events

[3vanlock]

Create a new alerter that sends Datadog Events via their API.

[nsano-rururu]

@3vanlock

I think I need to add a description of the datadog configuration to elastalert/docs/source/ruletypes.rst.
Since the first character is d, I think it is good to add it between command and email.

Example)

Datadog
~~~~~~~

Events are limited to 4000 characters. If an event is sent that contains a message that is longer than 4000 characters, only his first 4000 characters will be displayed.

datadog_api_key: Datadog API key.

datadog_app_key: Datadog Application key.

[3vanlock]

add a description of the datadog configuration to elastalert/docs/source/ruletypes.rst

Done @nsano-rururu. Thanks for the structure. I added a couple of links to Datadog's own Event documentation.

[jertel]

I see some of the other alerts have unit tests. Should a test, or tests, be added for this new logic?

[3vanlock]

Good call @jertel. I added a new test following the style of the old tests and ran it with success locally.