Handle scrollBehaviour with transitions

[posva]

from #1262

Vue.js / vue-router versions

2.1.2 / 2.1.1

Reproduction Link

http://jsfiddle.net/qw4hbgek/

Problem

Clicking on Home, then on post1 will not scroll down because of the transition

Feature request

We should at least provide a way of doing it userland in the docs. Ideally, scrollbehaviour should trigger once the new view is inserted into the dom

[homerjam]

Would supporting a returned promise here be enough to support most cases where there is a transition etc?

[homerjam]

I've had a go at this, check out this commit.

It could be improved, if there were away to obtain the transition duration from the route object for example.

Would love to get some thoughts/input @posva @atinux @pi0 @yyx990803