[Console][Translation] yaml output support for nested messages codes

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: #6192
Todo: -
License of the code: MIT
Documentation PR: -

`translation:update` command now accepts the option `--nest-level` which works in conjunction with YAML output format.
This options, which must have an integer value starting from 1, makes the dumper output nested YAML translation codes instead of line-by-line, as supported by the translator when using codes instead of real text in message files.

    admin.hello: Hello administrator!
    admin.exit: Quit administration

becomes:

    admin:
        hello: Hello administrator!
        exit: Quit administration