Generate gzipped assets.

Mincer used to gzip assets by default [1][2], but now requires options to be specified [3].

1: https://github.com/nodeca/mincer/blob/0.5.10/lib/mincer/manifest.js#L208-L216
2: https://github.com/nodeca/mincer/blob/0.5.10/lib/mincer/assets/asset.js#L247-L249
3: https://github.com/nodeca/mincer/blob/1.1.0/lib/mincer/manifest.js#L256-L258
It might also make sense to spin them out into different options.