[FrameworkBundle] [Routing] added option to generate route names using underscores

This allows generated route names to be separated by underscore instead of just converting them to lowercase.

Example:
StemerdinkIt\CamelCasedBundle

Would now be:
stemerdink_it_camel_cased_...
Instead of:
stemerdinkit_camelcased_...

To keep it backwards compatible it is not enabled by default. It can be enabled via config.yml:

framework:
    router:
        naming_strategy: underscore

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

This change also depends on a change in the SensioFrameworkExtraBundle: StemerdinkIT/SensioFrameworkExtraBundle@836cafd69addac9e6a82b81a42b5e06bac5f22a1