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
I'm sure I had a good reason to put it there when I submitted this last week but I can't for the life of me remember now. Looking at it now it doesn't appear to matter what order these error checks show up in, so I'll leave it the way it is in master.