Allow config to be read from a file?

[Andy Waite]

👋 I'm running htmlbeautifier with VS Code via https://github.com/aliariff/vscode-erb-beautify

The extension has its own configuration mirroring the flags for htmlbeautifier. I think it could be useful if htmlbeautifier was able to read from a simple config file, so that the command line tool could share the same config as editor extensions/plugins.

cc @aliariff