Implement a missing key strategy

[bratan]

Currently the key itself is returned if the localization is not found.

We should provide the option to return the fallback value instead.

Implement a MissingKeyStrategy enum with the following options:

• Key (default)
• Fallback

If Fallback option is selected but is not found then Key will be returned.

#34 #46 #93

[stale[bot]]

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