Support PlantUML via command line

Description

Gitlab has a nice PlantUML integration. Unfortunately this needs a PlantUML server running, which propably is unnecassery in most cases.

Proposal

Support calling PlantUML via command line.

It might work like this:

• Enable PlantUML cmd mode in gitlab.rb
• Reconfiguring Gitlab than might: Download the latest PlantUML jar and check if Java is installed and usable at the systems plattform.

Links / references

• https://docs.gitlab.com/ee/administration/integration/plantuml.html
• http://plantuml.com/starting