Allow to provide minifier (Terser) options

[莊喬]

Hi, I have some scripts use return on top level thus can't be converted.

Then I find we can tell Terser to enable bare_returns to accept it.
https://github.com/terser/terser#parse-options

Would it be good to add a CLI option like --minify to delegate to Terser?

I am willing to prepare a PR, maybe introduce yargs to help handling the options.