Add webhook alerter and docs

[lastbulletbender]

The webhook alerter uses jinja templating engine which allows creating user customizable endpoints and body.

[nsano-rururu]

@lastbulletbender

Maybe I need to add Jinja2 installation to requirements.txt and setup.py?
https://pypi.org/project/Jinja2/

[lastbulletbender]

@nsano-rururu Ah yes! Forgot to add it to the commit. I will do it now.

[lastbulletbender]

I don't usually write in python. So any and all feedback is appreciated!

[nsano-rururu]

Isn't it better to add example_rules as well?
https://github.com/Yelp/elastalert/tree/master/example_rules

Since there is a description about the alert type in README.md, shouldn't it be better to add "webhook" as well?
https://github.com/Yelp/elastalert/blob/master/README.md

Currently, we have built-in support for the following alert types:

Email
JIRA
OpsGenie
Commands
HipChat
MS Teams
Slack
Telegram
GoogleChat
AWS SNS
VictorOps
PagerDuty
PagerTree
Exotel
Twilio
Gitter
Line Notify
Zabbix

[nsano-rururu]

The test code seems to be written in the following file. It seems that travis-ci is running and the test code written in pytest is running when the pull request is made.
I don't have any knowledge about pytest because I haven't written Python.
https://github.com/Yelp/elastalert/blob/master/tests/alerts_test.py

[lastbulletbender]

I completely missed the example_rules and Readme. I'll add those. Thanks @nsano-rururu

I am currently in middle of writing tests for these as I haven't used pytest before this.

[nsano-rururu]

If you describe the parameter and data type in schema.yaml, an error will appear on the console when the rule runs with a value different from the specified data type, for example, specifying the integer value and specifying the string value. Because the data type of the parameter specified in the rule is different
https://github.com/Yelp/elastalert/blob/master/elastalert/schema.yaml

[nsano-rururu]

@lastbulletbender

Any way you could push this PR to https://github.com/jertel/elastalert2 ?

[lastbulletbender]

Hey @nsano-rururu ! Sure. Let me write up the tests which I had skipped earlier and add create a PR. I'll do it by end of next week.