Update asset-minifier to allow es6 minification #53

[rangeoshun]

This PR changes the interface a bit but solves #53.

Generally, it should be backward compatible, as in updates to packages are only needed in minify opts.

:lang -> :lang-in

The ES6 input should be still OK for all packages.

Notes, objections?

[rangeoshun]

I could rename lang-in to the old lang if that is more acceptable, but would lack the consistency.

[rangeoshun]

One note tho, the description for the language argument was saying that the default languga is ES3, but I think it was not true since a long time.