Let EmberApp.import() mount non-js/css assets at arbitrary location

I was having trouble getting vendor fonts into my project. I tried app.import but that only seemed to work for .js and .css files. I then used the workaround from #735, but that didn't seem very elegant, and I couldn't get it to work at all from inside an add-on. So I dug a little deeper to try and fix app.import for non-js/css files. This PR makes the following work:
```
app.import('vendor/bootstrap/dist/fonts/glyphicons-halflings-regular.woff', {
    destDir: '/fonts'
});
```
The path passed in `destDir` is relative to /assets, so the file will show up in the final distribution as `/assets/fonts/glyphicons-halflings-regular.woff`. The default is to put the file where the previous code looked to be trying to put it, at the relative path to the file from vendor/, so
```
app.import('vendor/bootstrap/dist/fonts/glyphicons-halflings-regular.woff');
```
would mount the file at `/assets/bootstrap/dist/fonts/glyphicons-halflings-regular.woff`.

I also moved the deprecation warning for options into a section that only applies to javascript files.

Thanks
:+1: - Using a local var for this repeated check is good.