ftl.lang: Add support for Mozilla's Fluent ftl file format

Syntax guide: https://projectfluent.org/fluent/guide/

Website: https://projectfluent.org

Repo for the spec: https://github.com/projectfluent/fluent

For a larger testfile see: https://github.com/projectfluent/fluent/blob/master/test/benchmarks/gecko_strings.ftl

I am having a hard time detecting multi-line blocks. Maybe related to this issue: #68 (closed)