Feature/assets install write to

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | This feature allow you to set up write path for your assets:install command. I did it as I found that there is no easy way to do write images to amazon s3 server. 

I writed this, because i tried to push my images to amazon s3 server and it was not possible with symfony. For now I created a bundle for this which create another assets:install command and write to amazon s3 server, but it will be great if this will be in symfony, for now you can do it with assets:install s3://key:secret@bucket but its not easy and hard to use with parameters.

With this change you only need to add
`````
framework:
    templating:
        write_to: s3://key:secret@bucket
`````

and your assets:install command will automagically send all your bundle public data to s3 or any other cdn.