GitLab CI: Add EditorConfig Validation

I've integrated EditorConfig validation in the GitLab CI build, using a separate job. I've tested it thoroughly on my fork here on GitLab, with and without errors, and it's working fine.

I've used a custom script to carry out the validation (validate.sh) in order to produce less verbose error reports, by listing only the culprit files in case of failure (whereas EClint would also list every error, producing hundreds of lines even for a single file). Also, this script can be used locally to validate contents before pushing them and making a MR (the script will print instructions on how to setup the toolchain in case of missing dependencies).

I think it's a good idea to also validate the code style of contributed commits, to keep the project consistently formatted.

The syntax tests script produces an extremely verbose log in case of error (tested it on my fork), it might be a good idea to somehow limit the failure report to the list of files which didn't pass the test (i.e. the langDef and the syntax tests in question).

As mentioned in !122 (merged), I'd like to improve automation and interoperability between this repository on the HL Test Suite, and to also add Travis CI settings for the GitHub clones (the two won't cross paths, and for the status badges we can benefit from Asciidoctor preprocessor directives to show the correct status badge based on the platform).