Automatic fix reference path in output .d.ts files

For https://github.com/kotas/gulp-tsc/issues/26

In output .d.ts files, tsc uses a temporary directory as a base for <reference> paths, which is wrong when the output directory does not have the same level as the temporary directory.

To fix this, we introduce an output filter to fix reference paths for output .d.ts files only. Same as fixing sourcemaps, we need `outDir` is specified to know destination paths.