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.