Support for Mozilla's Fluent Localization Framework

[Former user]

Mozilla has introduced a new l10n framework, Fluent (https://projectfluent.org). The localizable strings are stored in a new file format, FTL. It is, basically, key-value based, but supports, in addition, attributes and alternatives (for multiple plurals, etc.) per translation unit (similar to PO files).

Currently, only Pontoon, Mozilla's own TM system, supports Fluent files (see https://blog.mozilla.org/l10n/2019/04/11/implementing-fluent-in-a-localization-tool/).

It would be great if Okapi provided a filter for FTL in order to enable other TM tools to translate Fluent files.