[FrameworkBundle] Added configuration for additionnal request formats
feature #9862 [FrameworkBundle] Added configuration for additionnal request formats (gquemener)

This PR was squashed before being merged into the 2.5-dev branch (closes #9862).

Discussion
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[FrameworkBundle] Added configuration for additionnal request formats

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/8934
| License       | MIT
| Doc PR        |  https://github.com/symfony/symfony-docs/pull/3402

Reopening of https://github.com/symfony/symfony/pull/8944

# TODO
  - [x] Fix wrong xml configuration definition (Thanks @WouterJ)
  - [x] Change configuration key `additional_formats` to a more meaningful one
  - [x] Write documentation (new entry or replace http://symfony.com/doc/current/cookbook/request/mime_type.html ?)

Commits
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f90ba11 [FrameworkBundle] Added configuration for additionnal request formats