encodebin: Add formatters support

Submitted by Thiago Sousa Santos @thiagossantos

Link to original bug (#649841)

Description

The attached patch is an initial attempt at autoplugging formatters for
each stream on the profile.

This patch makes it pick the first formatter it finds, testing it with
an mp3 profile and bumping xingmux rank made it pick it successfully.

One decision to make is how many formatters are we going to plug? All
available? First found? Best rank?