Included sample and extension .ML extension for Standard ML files

Standard ML code (particularly the Poly/ML dialect) often has the .ML file extension. Probably the most prevalent example is in the theorem prover Isabelle. See:
* http://isabelle.in.tum.de/repos/isabelle/file/1d122af2b11a/src/Pure

Examples on github:
* Quantomatic/quantomatic
* Quantomatic/isaplib
* ggrov/psgraph