Add input plugin for rollbar service. (#1247)

* Report rollbar events.

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

* Fix indent with go fmt.

* Add test for rollbar webhooks.

* Report more data from new_item event.

* Handle new deploy webhook.

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

* Update default port.

* Add readme.

* Add rollbar_webhooks to the readme.

* Add rollbar_webhooks to plugins list.

* Add tag level for new_item event.

* Update readme.

* Update changelog.