tool ui: add stats for savings after gzip

[stoyan]

Original size: X. Minified size: Y. Savings: Z (ZZ.ZZ%).
Gzipped original size: A. Minified size: B. Savings: C (CC.CC%).

[kangax]

Only took us 4.5 years but this is now done :) thanks to @duncanbeevers

[duncanbeevers]

And @rsp!