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.Dockerfile

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