feature request: allow multiple file-extensions in CLI

I'm using html-minifier to minify all the .html and .json files (in my case they include some html markup) in a directory.
Right now it doesn't seem to be possible to give multiple file extensions via `--file-ext` so I have to call `html-minifier` at least twice.
It would be great to be able to either specify allowed file extensions in the config file or to allow specifying several file extensions vie CLI.

Start by locating the CLI handling for `--file-ext` and the configuration-file option parsing. Verify how a single extension is currently accepted, then ensure multiple extensions can be supplied through the CLI or configuration and confirm that both `.html` and `.json` files are processed.

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