Issue title: add command: jx trigger nameOfPipeline
 Issue body: it would be nice to have a command line that can be used to trigger a pipeline build on the current jenkins.

Something like...
```
$ jx trigger foo
Pipeline foo has started
```

Maybe if no pipeline name is specified then we could show a combo box list of options to select (using the `survey stuff`) 
e.g.
```
$ jx trigger 
Please select the pipeline to trigger
> [x]: pipeline1
   [  ]: pipeline2
   [  ]: pipeline3
```


 Comments: 
 Comment 0: am not sure if the command should be `trigger` or `start` though - I guess we could add an alias for both ;)
{'source': 'GithubIssues', 'repo_name_with_owner': 'jenkins-x/jx', 'issue_id': 288166512, 'repo_stars': 4336.0}