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.
Hmm, that's interesting. `f8f8f8` is used for code-style text background and in another place, but I don't think any of the usages are related/should be refactored. Same for the border color. I think the CSS could use cleanup/more variables, but I don't think it's worth it to do along with this just based on looking around.