Paramaterize the DOM selector for the app's mount point

[jgaehring]

In app.js, the main load function has #app hardcoded as the DOM selector on which the Vue app is mounted. That is, it's return value is the following Vue instance:

new Vue({
    el: '#app',
    store,
    router,
    components: {App},
    template: '<App/>'
  })

Ideally, that selector should be parameterized, so whatever implementation that's consuming the client library can decide where in the DOM to mount the app, what to name that mount point, etc, etc.

This is a pretty small issue, but I'm just leaving it here for now, because it's not necessary to the next stage of feature development on the native app, and it lies tangential to much broader issue of how to generalize the load function further, how to allow more plugins to be accepted as parameters, and whether or not to decouple Vue from the core logic of the load function. So I'm kicking this small can down the road for now.

[mstenta]

(Transferring all issues from old repository. See #92)