Added feat to write to different output file.

[Symbitic]

I know in issue #29 you said you didn't want this feature added, but please hear me out.

I added this because I'm trying to port Babylon.js to Rollup. The core module alone takes 2 minutes for the UMD bundle, and the minified UMD bundle takes another 2 min and 30 seconds. I can majorly cut that down by only generating the bundle once and then just writing normal and minified to different files.

The only change is that terser({ filename: "custom.js" }) will write to "custom.js" and leave the regular output file untouched. It's literally just a few extra lines of code (just adding an extra promise at the end), and all tests still pass.

Please consider this.

[TrySound]

Hi, thanks for this. Though I think such feature fits better into pipeline and may spend less time than duplicated build TrySound/rollup-plugin-uglify#61

[Symbitic]

Can you please explain how that pull request relates to this?
The whole point of this is to avoid duplicated builds if a minified and non-minified bundle are desired.

[TrySound]

Build itself will not be duplicated. Only generation will be called twice with this feature.

[Symbitic]

I think I understand. So I'm gonna close this. Thanks for your answers.