Prefer icons.js for subsetting icons

[jrjohnson]

Storing long lists of icons in environment.js results in very large meta
tags for the application. Since these have to be parsed before content
can be shown this can slow down the loading of boot animations that are
intended to load before the application is parsed.

Fixes #112

[acorncom]

This looks great. Unless we should stick with the Ember standard of putting this config in ember-cli-build which is generally where build transforms go?

[jrjohnson]

@acorncom I'd prefer it there as well, but consolidated on ember-cli-build in #68 as getting build time options into the component where they are needed for determining the default icon set was too difficult for me to figure out.