Add support for generating hash keys based on all files in a glob's base directory.

I want to use AMD in my stack on the client side, in order to make things smooth I want all my generated resources to be placed in a unique directory based on their contents.

gulp-buster was really close to being able to do this so rather than reinvent the wheel I wanted to add a few lines that will add a new configuration property: "mode"

if mode is set to 'dir', the generated hashes will be based on the contents of all files that match the src glob, while their key will be the file.base of that glob.

So something like:
```
var bust = require('gulp-buster');
gulp.task('cache-bust', function() {
  bust.config({mode: 'dir', fileName: 'buster.json'});
  return gulp.src(['static/src/**/*.*', 'views/src/**/*.*'])
    .pipe(bust())
    .pipe(gulp.dest('.'));
});
```

will result in:

```{"static/src":"ad8ef4e7644e3ebd7b902136ed9ea2b4","views/src":"1e2cf10132a084e8005b162dd9f51151"}```