Allow other file extensions

[strarsis]

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

make-pot should allow configurable extension for the files to scan.
E.g. when the theme uses the blade template language (like the roots.io Sage theme), many files will have the extension blade.php instead just php. Currently make-pot ignores these and the translations strings in the POT file are missing.

Describe the solution you'd like
A configuration option for the make-pot command for specifying different/more extension of the files to scan.

[strarsis]

Actually for support of files with multiple file extensions (as blade.php for PHP-Blade files),
the IterableCodeExtractor filter has to be refactored, see this PR for PHP-Blade templates: #304

[swissspidy]

Closing this since dedicated Blade support was added in #304.

If it turns out that such configuration is needed for other file types, we can reopen it.

Edit: see also #231