Include ActiveSupport and refactor accordingly

[jaredcwhite]

I've come to discover I'm instinctually an "ActiveSupport-flavored" Rubyist, so not having all the niceties that come from ActiveSupport is a real bummer. I've already identified a number of places where both the Bridgetown internals and external plugin APIs could be improved substantially from utilizing ActiveSupport, so I think it's time to bite the bullet and get this added. I'm not sure if it will add a small amount of extra boot time, so that's something to consider, but if we're talking 100ms or less, I'm not too worried.

(Another good reason to add ActiveSupport: a major goal for this project is to find symbiotic ways for Bridgetown and Rails to work together and swim in the same developer communities, so the less friction there is for Rails folks to get up to speed with Bridgetown, the better.)

[jaredcwhite]

Note: changing milestone to 0.14 due to using HashWithIndifferentAccess for upcoming plugin DSL stuff, but really the bulk of the work to implement this will be in 0.15 and beyond.