Add new webhooks plugin that superseed github and rollbar plugins.

closes #1289

Signed-off-by: François de Metz <francois@stormz.me>
Signed-off-by: Cyril Duez <cyril@stormz.me>

Rename internals struct.

Signed-off-by: François de Metz <francois@stormz.me>
Signed-off-by: Cyril Duez <cyril@stormz.me>

Update changelog.

Signed-off-by: François de Metz <francois@stormz.me>
Signed-off-by: Cyril Duez <cyril@stormz.me>

Update READMEs and CHANGELOG.

Signed-off-by: François de Metz <francois@stormz.me>
Signed-off-by: Cyril Duez <cyril@stormz.me>

Update SampleConfig.

Update the config format.

Update telegraf config.

Update the webhooks README.

Update changelog.

Update the changelog with an upgrade path.

Update default ports.

Fix indent.

Check for nil value on AvailableWebhooks.

Check for CanInterface.

plugins/inputs/webhooks/README.md

@@ -0,0 +1,27 @@
+# Webhooks
+
+This is a Telegraf service plugin that start an http server and register multiple webhook listeners.
+
+```sh
+$ telegraf -sample-config -input-filter webhooks -output-filter influxdb > config.conf.new
+```
+
+Change the config file to point to the InfluxDB server you are using and adjust the settings to match your environment. Once that is complete:
+
+```sh
+$ cp config.conf.new /etc/telegraf/telegraf.conf
+$ sudo service telegraf start
+```
+
+## Available webhooks
+
+- [Github](github/)
+- [Rollbar](rollbar/)
+
+## Adding new webhooks plugin
+
+1. Add your webhook plugin inside the `webhooks` folder
+1. Your plugin must implement the `Webhook` interface
+1. Import your plugin in the `webhooks.go` file and add it to the `Webhooks` struct
+
+Both [Github](github/) and [Rollbar](rollbar/) are good example to follow.