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

In an addon, if you use `app.import()` to pull in a non-js/css asset from `vendor/`, ember-cli will fail to find the file unless it exists in the application's `vendor/` subdirectory. For example, if you have an addon called `ember-cli-bootstrap` that has the following in its `included` hook:
```
app.import('vendor/bootstrap/dist/fonts/glyphicons-halflings-regular.woff');
```
then if you run `ember build` you will get the following error:
```
Error: Path or pattern "glyphicons-halflings-regular.woff" did not match any files
  at Object.multiGlob (/Users/csantero/Documents/Projects/ember-cli/node_modules/broccoli-static-compiler/node_modules/broccoli-kitchen-sink-helpers/index.js:221:13)
```
This error does not occur if your application has a file at `vendor/bootstrap/dist/fonts/glyphicons-halflings-regular.woff`, but without the fix addressed by this PR, it is impossible to have an addon that installs non-js/css assets without relying on bower.

This fix made it easy to add an additional enhancement that allows `app.import` to specify the destination path of non-js/css assets. 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`. If no `destDir` is provided, the asset will be placed at its relative path 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 would prefer to just pass trough the destDir (aka not prefix with assets). This offers much better flexibility, and since it is a new feature we won't have backwards compat issues.