Provider reference passing update

## Background
The ability to pass an entity reference to a provider was added in nelmio/alice#50, however the regex checks for boundaries, which results in limiting these references to a single occurrence.  The inside of a provider call can contain arbitrary PHP so this seems unnecessary.

## Implementation
I added 935e138 so that I had a provider available with which I could write a test for 36ba7c5.  Otherwise it is no necessary for this fix.  However, we have found the `identity` provider incredibly useful paired with the ability to perform simple operations such as math inside the provider call.  Some framework was added to support having built-in providers.

It is made more convenient by adding some syntactic sugar, `<()>` is the same as `<identity()>`.