Allow regular expressions in proxies definition

### Wished behavior

I would like to be able to tell Karma to serve a file whatever the sub-path is.
I could also want to transform the URL to something completely different.
For instance, something like this (but it won't work with the object syntax):

``` javascript
proxies: {
/\/images\/.*/: "/base/test/assets/placeholder.png",
/\/books/(\d+)\/pages\/(\d+)/: "http://external.host/get_book_page.php?book_id=$1&page_id=$2"
}
```
### Proposed syntax

`proxies` could accept an object, as it is currently the case, or an array of arrays, which is still readable, and accepts regular expressions as "keys":

``` javascript
proxies: [
[/\/images\/.*/, "/base/test/assets/placeholder.png"],
[/\/books/(\d+)\/pages\/(\d+)/, "http://external.host/get_book_page.php?book_id=$1&page_id=$2"]
]
```