how to custom sourcemap filename

hi, @rickharrison

now , in ember-cli config , i can generate hash files with sourcemap files, like this:
```
vendor-fce6de4e72c198ba468a0eb2c83d1fb4.js
vendor-637ec5408a9ef70307a43842b526828d.map
```

but, we hope the mapfile name is same as origin js file . can `broccoli-asset-rev` support custom map file name like this:

```
vendor-fce6de4e72c198ba468a0eb2c83d1fb4.js
vendor-fce6de4e72c198ba468a0eb2c83d1fb4.js.map
```

thanks

Start by tracing how broccoli-asset-rev generates the fingerprinted JavaScript and sourcemap filenames when used through ember-cli config. Compare the current differing names with the requested matching .js.map name; done means the sourcemap can use the JavaScript asset's fingerprinted filename.

Written by the indexing model from the issue text.