Feature Request: Declaritive trigger for notes (in merge requests)

## Issue
When running a freestyle build, this plugin provides a field titled "Comment for triggering a build" that consumes the notes in a merge request and, if they match the provided regex, starts a new jenkins build against the same commit.

There doesn't appear to be the same feature in the pipeline declarative syntax (unless I'm missing it...if so then this issue could be re-directed to simply improve the documentation), or the procedural syntax for that matter.

In the declarative example in the readme there is a section:

triggers {
gitlab(triggerOnPush: true, triggerOnMergeRequest: true, branchFilterType: 'All')
}

To solve this issue, we could just update it so that the following works:

triggers {
gitlab(triggerOnPush: true, triggerOnMergeRequest: true, triggerOnNote: 'regex to use', branchFilterType: 'All')
}

### Context
- **Gitlab plugin version**: 1.4.5
- **Gitlab version**: CE 8.17.0
- **Jenkins version**: 2.46.1
- **Job type**: Pipeline