Changes/Additions to Jenkins command

[garethr]

The current Jenkins commands are pretty tailored to a specific Jenkins setup:

• They assume you want to pass a branch to a job (and mandate how)
• They don't allow triggering of jobs without parameters
• They don't allow the passing of more parameters
• They prioritise a branch name over the job, as if you'll only really care about a single job

This pull both adds a new list jobs command and tweaks the build command to be more flexible.

• You can build any job
• You can specify any number of parameters

The downside is it's backwards incompatible, and the very specific usecase of the existing command goes from build branch master to jenkins build BRANCH_SPECIFIER=master.

I think this approach is more generally applicable to a shared repo, but then I would wouldn't I. Happy to split this off into another file with a different name so it doesn't clash if preferred. Wasn't sure on the etiquette of multiple files for a given application.