add command: jx trigger nameOfPipeline

[jstrachan]

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 and there are > 1 pipelines then we could show a combo box list of options to select using the survey stuff - like this code: https://github.com/jstrachan/jx/blob/033629a9e2c565fb887b61f2d9305479abf44a06/pkg/auth/config.go#L175-L184

e.g.

$ jx trigger 
Please select the pipeline to trigger
-> [x]: pipeline1
   [ ]: pipeline2
   [ ]: pipeline3

[jstrachan]

am not sure if the command should be trigger or start though - I guess we could add an alias for both ;)