Gathers files that will be modified.

@type {Object} config - A configuration.
@return {Promise} A promise that returns an array containing the given configuration and the identified files.
function getFiles(config) {

	return new Promise((resolve, reject) => {

		const src = config.src;
		let files = [];
		let i = 0;

		(function proceed(error, moreFiles) {

			if(error !== null) {

				reject(error);

			} else {

				files = files.concat(moreFiles);

				if(i === src.length) {

					if(files.length === 0) {

						reject("No input files found");

					} else {

						resolve([config, files]);

					}

				} else {

					glob(src[i++], proceed);

				}

			}

		}(null, []));

	});

}