Ooops, CLI does not work when html-minifier is installed with npm install html-minifier -g

Today I installed html-minifier on a Mac with `npm install html-minifier -g` command and noticed that the CLI interface did not get installed at all... :(

Here is the fix: we must include `cli.js` in the `"files"` section of `package.json` file. It's kind of important.

While at it, I did some other improvements in my cli.js code, fixed a small issue in README.md file, etc.

Can you please merge these changes?

Thanks!!