New column resizing and configurable column demo

Splits resizing behavior into two modes (`normal` and `fluid`).

@bigsley @Dylnuge @igillis - wondering about the right API for this second mode. I decided against `fixedWidth` in the end, because it really has nothing to do with the width of the table and everything to do with how columns interact. I'm also debating between the API here (`fluid: yes/no`) and a mirror of `selectionMode` (`columnResizeMode: 'normal'/'fluid'`).

For the new changes, see the spec: https://github.com/Addepar/ember-table/issues/175, and previous review: https://github.com/Addepar/ember-table/pull/176.

Normal mode works as before.

Fluid mode makes columns steal width from their neighbors when they resize.

To see how this all works, I've added a Configurable Column Demo that allows you to play with these options and see a demo table respond.

@cyril-sf there's a hack in here I'd love for you to take a look at. Will add a comment.
@bigsley @Dylnuge @igillis @sreenath-addepar could all potentially be interested in reviewing.
Same thing. It also looks like there's some code duplication in our example app, but that's probably not the worst thing (and it's not this PR's responsibility to clean it up, just noting it)