Support multiple extensions on --file-ext

[yumin-chen]

This pull request adds support for multiple extensions on --file-ext.

For example:

html-minifier --input-dir _site --output-dir _dist --file-ext html,htm,json,xml,yml,yaml

This can filter out all supported extensions html,htm,json,xml,yml,yaml in one go.

[yumin-chen]

Addresses issues: #896 feature request: allow multiple file-extensions in CLI

[alexlamsl]

@kangax very sorry to bother you again, but looks like Travis can't build PR branches from forked repositories 😅

https://travis-ci.com/kangax/html-minifier/requests

[alexlamsl]

@kangax so I think given all the recent hassle, it's best if we:

• enable all webhooks for Travis CI
• disable whatever "branch protection" settings on GitHub

... since it's better to have some progress than none at all.

[alexlamsl]

@kangax any luck?

[kangax]

@alexlamsl sorry, been travelling. Wish I could just give you access to travis. I'll try to get to it today/tomorrow.

[yumin-chen]

@kangax @alexlamsl Any update?