Add GitLab CI

Depends on #2 (closed).

• Use CI Utils repos, add as submodule
• debian:testing image with required packages installed
  • Refer to #1 (closed) for information
• Build and test using both gcc and clang
  • Means you have 2 stages for build, e.g. build:gcc and build:clang
  • View GitLab help for more information