Cache remote downloads and .gitlab-ci.yml

**Is your feature request related to a problem? Please describe.** I am using gitlab-ci-local in multiple bash scripts, and I'm tired of waiting almost one second each invocation for seemingly doing the same work everytime. Example: ``` $ gitlab-ci-local --list parsing and downloads finished in 941 ms. json schema validated in 132 ms ``` **Describe the solution you'd like** Cache of remote fetches and/or .gitlab-ci files using checksums **Describe alternatives you've considered** **Additional context** It could be made opt-in (or at least opt-out) to avoid debuggin stale caches.