exclude external paths option

[AVCEngineering]

adds an option to exclude external paths such //cdn.js/jquery.1.14.js. Option is false by default. Fixes #4

Usage:

gulp.task( 'default', function() {
	gulp.src( [ 'source/*.*' ] )
		.pipe( pathPrefix( { prefix: '/analytics', excludeExternal: true } ) )
		.pipe( gulp.dest( './result' ) );
} );