Add support for multiple files

Keeps the latest `keepQuantity` rev'd versions of each unique file instead of the latest version of all globbed files. Eg: 
```javascript
var gulp         = require('gulp');
var revOutdated  = require('gulp-rev-outdated');
var cleaner      = require('gulp-rimraf');

gulp.task('clean', function() {
    return gulp.src( ['dist/js/{vendors,bundle}*.js'], {read: false})
        .pipe( revOutdated(1) )
        .pipe( cleaner() );
});
```
The above will have the same effect/output as

```javascript
var gulp         = require('gulp');
var revOutdated  = require('gulp-rev-outdated');
var cleaner      = require('gulp-rimraf');

gulp.task('clean', function() {
    gulp.src( ['dist/js/vendors*.js'], {read: false})
        .pipe( revOutdated(1) )
        .pipe( cleaner() );

    gulp.src( ['dist/js/bundle*.js'], {read: false})
        .pipe( revOutdated(1) )
        .pipe( cleaner() );

    return;
});
```
It'll keep the newest `vendors-<rev>.js` as well as the newest `bundle-<rev>.js`.