Added opengraph handler for keymedia field type, which can be used with NetgenOpenGraphBundle

[iherak]

This PR adds opengraph handler for keymedia field type.
This handler is used with NetgenOpenGraphBundle to configure and dynamically create opengraph tags, per content type.

Usage is similar to other field type handlers defined in https://github.com/netgen/NetgenOpenGraphBundle/blob/master/Resources/doc/USAGE.md

• { handler: field_type/keymedia, tag: 'og:image', params: ['image', 'Large'] }

First parameter is the field type identifier, the second parameter is the format (named or not named). Both parameters are required.