feature request: allow multiple file-extensions in CLI

[basbebe]

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.

[yumin-chen]

I've created a pull request to support multiple extensions on --file-ext #1026