Add support for several other file types

Adds support for Dockerfiles, Gemfiles, Ruby files, and Groovy files.
Also, in order to fully support Dockerfile and Gemfiles, a new file
extension parser was created to deal with files that do not have an
extension.

testdata/initial/file.groovy

@@ -0,0 +1,3 @@
+def dummy_function() {
+    println "Hello world"
+}