Add fallback formatting

[sebwas]

This pull request aims to add formatting of fallback messages if that's desired (it's completely controllable by the i18n options and the default is to turn it off (so that it's 100% backwards compatible))

Turn it on by setting the formatFallbackMessages to true.

The use case is simple: When using the whole string to be translated as key, you will want to format the messages nonetheless:

<div>
  {{ $t('Hello {name}!', { name: 'world' }) }}
</div>

[codecov-io]

Codecov Report

Merging #637 into dev will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              dev    #637      +/-   ##
=========================================
+ Coverage   96.18%   96.2%   +0.01%     
=========================================
  Files          10      10              
  Lines         787     791       +4     
=========================================
+ Hits          757     761       +4     
  Misses         30      30

Impacted Files	Coverage Δ
src/index.js	98.05% <100%> (+0.02%)	⬆️

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[sebwas]

Any comments, @kazupon?

[kazupon]

@sebwas
Sorry, late reply. 🙇

That's a very interesting feature request!
If the specified key cannot be found, it's convenient to interpolate.

We can use that feature for key like message (e.g. This is {name}). We cannot use for key like path. We need to notice to use this feature.
Some documentation updates are required to provide this feature.
Can you update docs? 🙏

[kazupon]

https://github.com/kazupon/vue-i18n/pull/637/files#diff-1fdf421c05c1140f6d71444ea2b27638R266
We need to change from _warnDefault , due to have some behavior.

[sebwas]

I don't exactly understand what you mean. Do you mean we should change the wording here, or?

[AlexandreBonaventure]

Hello what about this issue #296 ? I think this would be worth to reopen, can I submit a PR ?