ls = ( inputDir ) => {
	return new Promise( ( resolve, reject ) => {
		fs.readdir( inputDir, ( err, list ) => {
			if ( err ) {
				return reject( err );
			}

			resolve( list );
		} );
	} );
}
Get the list of files in a directory, either as a list of file and subdir
names or a list of absolute file system paths

@private
@param {string} inputDir The file system path to the directory to read
@returns {Promise} A promise to the string array of file names