New feature: $createObservableFunction

[regou]

Convert function calls to observable sequence which emits the call arguments.
This make vue-rx become more collaborative with components that using callbacks

[regou]

@yyx990803 It's been a while... Will you review this or should I merge directly ?
My fork is already on our production environment.
It'll be great if the community able to use it after the next release.

The usage is like this:
In template, normal callback
In constructor.
Maybe can save you some time.

[yyx990803]

@regou thanks for the PR! This one slipped through the notifications.

I've got some feedback regarding the API: when I see a method being called in the template, I tend to look for it inside the methods block, but since it's registered imperatively, it can technically appear anywhere (lifecycle hooks, subscriptions function...) So this makes it less obvious to locate where a method is defined.

Maybe similar to the domStreams option, we can have an observableMethods option that makes it more declarative, which just calls $createObservableMethod internally (btw, $createObservableMethod seems more consistent with methods).

[regou]

@yyx990803 5c8f9c3
With simplified example