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/Gemfile

@@ -0,0 +1 @@
+puts "Hello world!"

testdata/initial/file.Dockerfile

@@ -0,0 +1,2 @@
+FROM scratch
+CMD ["echo", "hello world"]

testdata/initial/file.groovy

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

testdata/initial/file.rb

@@ -0,0 +1,2 @@
+# frozen_string_literal: true
+puts "Hello world!"