Modifies i18n_usage check to consider explicit default textdomain usage a warning

[felixarntz]

Fixes #713.

Note: There's no way to do this via WPCS flags, so we need to rely on the error message to detect the relevant violation and downgrade it to a warning accordingly. This can be done with minimal additional code by overriding the relevant parent method.

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[swissspidy]

Still not a fan of projects doing this but this is better than the current experience I suppose.

[ernilambar]

Still not a fan of projects doing this but this is better than the current experience I suppose.

Agree. Personally I would want my plugin's string independent from core string and have full control to the plugin itself.