exclude 'web' polyfills from node by default

[mshustov]

Hi, after running babel-env-preset with useBuiltIns: true for node (v7.6) environment got in console:

Using targets:
{
  "node": 7.6
}

Modules transform: false

Using plugins:
  syntax-trailing-function-commas {"node":7.6}

Using polyfills:
  es7.string.pad-start {"node":7.6}
  es7.string.pad-end {"node":7.6}
  web.timers {"node":7.6}
  web.immediate {"node":7.6}
  web.dom.iterable {"node":7.6}

So, I was surprised to find 'web' polyfills included in the final node build (I use webpack for both node and web). Of course I could use exclude option, but probably it should be default behaviour for node env.

[hzoo]

Sure we can do that. Do you want to make a PR?

[existentialism]

Fixed by #226