Refactored text asset filter to use dynamic binding

In order to allow middleware (e.g. handlebars, sass) to work with text files more easily, e.g.:
```clojure
(binding [assets/*text-asset?* (partial re-matches #".+\.(:?js|html|hbs)$")]
  (assets/load-bundles "assets"
                       {"app.js" [#"/javascripts/.+\.js$"
                                  #"/templates/.+\.hbs$"]
                        "style.css" [#"/stylesheets/.+\.css$"]}))
```