Add npm scripts
Add npm scripts

Especially useful for running tests (I struggled with running tests
when first contributing).

Adds:
- `$ npm test`
- `$ npm run lint`
- `$ npm run build`
- `$ npm run dist`

Since `$ npm test` wasn’t defined, and there was a gulpfile, I then
tried `$ gulp test`. That didn’t work, so I had to open and scan to
figure out it’s `$ gulp unit`. Not the best experience for new
contributors.

YOTOV-LIMITED/incremental-dom,PolymerLabs/incremental-dom,sparhami/incremental-dom,jridgewell/incremental-dom,reggi/incremental-dom,paolocaminiti/incremental-dom,google/incremental-dom,google/incremental-dom,sparhami/incremental-dom,loverajoel/incremental-dom,google/incremental-dom,sparhami/incremental-dom,YOTOV-LIMITED/incremental-dom,markuskobler/incremental-dom,thejameskyle/incremental-dom,PolymerLabs/incremental-dom,loverajoel/incremental-dom,sparhami/incremental-dom,reggi/incremental-dom,jridgewell/incremental-dom,thejameskyle/incremental-dom,PolymerLabs/incremental-dom,jridgewell/incremental-dom,paolocaminiti/incremental-dom,Rich-Harris/incremental-dom,Rich-Harris/incremental-dom,markuskobler/incremental-dom