add get-compiled-template function

eventually this will handle fetching pre-compiled templates out of the
cache, but for now this will help us refactor everything else to use
this instead and work with compiled template objects instead elsewhere

src/clj_jtwig/core.clj

@@ -13,6 +13,13 @@
 ; be rendered by calling it's 'render' method
 (defonce compiled-templates (atom {}))
 
+(defn- get-compiled-template [file]
+  (if-not (.exists file)
+    (throw (new FileNotFoundException (str "Template file \"" file "\" not found.")))
+    (->> file
+         (new JtwigTemplate)
+         (.compile))))
+
 (defn- create-function-repository []
   (new DefaultFunctionRepository (make-array JtwigFunction 0)))