[Console] Added standalone PSR-3 compliant logger
feature #10194 [Console] Added standalone PSR-3 compliant logger (dunglas)

This PR was merged into the 2.5-dev branch.

Discussion
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[Console] Added standalone PSR-3 compliant logger

This PR adds a standalone, PSR-3 compliant, logger to the Console component. It logs all messages on the console output. Messages of `DEBUG`, `INFO` and `NOTICE` levels are displayed using the `info` format (default to green). Higher levels are displayed using the `error` formatter (default to red).

This logger is similar to the [Monolog's Console Handler](http://symfony.com/doc/current/cookbook/logging/monolog_console.html) but does not have any external dependency (except `php-fig/log`). This is useful for console applications and commands needing a lightweight PSR-3 compliant logger (e.g. required by a dependency or to display basic informations to the user).

An usage example is available here: https://github.com/dunglas/php-schema.org-model/blob/master/src/SchemaOrgModel/Command/GenerateEntitiesCommand.php#L71

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#3696

Commits
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e40b34d [Console] Added standalone PSR-3 compliant logger