Fix syntax detecting (#99)

Get file's extension and check if it's present in a list of supported syntaxes.
If not, print an error.

![tumblr_msq32tmwsc1ssucqko1_400](https://f.cloud.github.com/assets/872004/1491460/397427a4-47af-11e3-877d-c5e6b683325d.gif)