Enable graceful fallback handling.

[nigeleke]

1. When a language_identifier does not have a direct translation in the config, it will be determined if one exists for the primary_language. If so, then that will be used, otherwise revert to the configured fallback. Eg: config contains "en" and "en-GB" but user has selected "en-US". "en-US" will not be found, so "en" will be used.
   The graceful fallback hierarchy follows -<script>--, rolling from the RHS back to .

2. It is possible to "share" a translation resource:

config
.with_locale((langid!("en-GB"), include_str!("./en-GB.ftl")
.with_locale((langid!("en"), include_str!("./en-GB.ftl")

where the included strings are not duplicated in the final config.

3. Other changes mentioned in CHANGELOG.

4. Note: v# bumped to 0.4.0

Closes #10

[marc2332]

Just had a quick look and all I have to say is this is impressive work! Feature well explained, added tests, comments, and docs, very nice job!

I'll have a longer look asap! 🦀

[nigeleke]

All comments taken onboard. @marc2332 this is my first formally reviewed PR, so not sure if you're expecting me to do a "completed changes" notification. I can't see any obvious action here on GH.
Otherewise - over to you.