Added options object that allows the user turn off the output file hash,...

... and also include a version string in the filename.

Tests pass.

  ✓ should rev files
  ✓ should not rev files if hash is false
  ✓ should rev and version if version set
  ✓ should not rev but add version if version set and hash is false
  ✓ should build a rev manifest file
  ✓ should allow naming the manifest file
  ✓ should append to an existing rev manifest file
  ✓ should respect directories
  ✓ should store the hashes for later

  9 passing (17ms)
@splars Most of my gulp tasks have a section that looks something like:

```js
      if (process.env.RACK_ENV !== 'production') {
        return scripts
          .pipe(gulp.dest('public/assets/js'))
          .pipe(livereload()));
      }
```

… just before the bit that minifies and revs assets.