…s no transform found for a file extension.

Currently this behaviour can only be reached when trying to instrument a single file.
In the case of instrumenting a directory, files with an extension with no matching transform are filtered out before they can be instrumented.

The main aim of this has been to clarify whether we're working with relative or absolute file paths, and removing unnecessary transformations.  Although I've made a few other 'small' changes here and there.

Key changes:
	* Created a new private method `NYC._transform`, common to `_maybeInstrumentSource` and `instrumentAllFiles`.
	* Renamed the param in `walkAllFiles` forEach handler to `relFile` to explicitly state the file representation being used.
	* Let the `addAllFiles` visitor function rely on `testExclude` to determine which files to instrument