Fix syntax detecting (close #99)

Get file's extension and check if it's present in a list of supported syntaxes.
If not, print an error.
Tests: Add tests for public methods
Remove syntax check from `processDirectory` method

There is already a syntax check inside `processFile` which is called for every
file in the directory, so there's no need to make the same check twice.
Tests: Add processFile(unsupported extension) case

Test passing a file with unsupported extension to `processFile()` method