Add blank line to output for FileDownloader.

The VcsDownloader outputs a blank line between each dependency:

https://github.com/composer/composer/blob/745dcbce3317f7119575c39cef2cb601f9c5ffcf/src/Composer/Downloader/VcsDownloader.php#L80

This write makes output consistent.
Fix missing validation on default value for author input

The default author value on the composer init command is not validated against `parseAuthorString` method and thus not being re-prompted, finally throwing an InvalidArgumentException when it tries to generate the composer.json file.

The changes forces the validation of both the entered author string or the default value.
Add some examples for "composer" type repositories.

These can be used as examples or possibly for tests.
added $installer->isDryRun()
added $installer->isVerbose()
Update fixtures to include gar/nix (which is being replaced) and bar/baz (which has a branch-alias)
Merge pull request #3122 from garthkerr/patch-1

Add blank line to output for FileDownloader.
Merge pull request #3319 from desyncr/bugfix/missing-default-author-value-validation

Fix missing validation on default value for author input
Merge pull request #3347 from webfactory/add-some-fixtures

Add some examples for "composer" type repositories.
Merge pull request #3348 from jakoch/patch-isdryRun

added $installer->isDryRun()
Merge pull request #3352 from phansys/composer_version

Added branch-alias to version output when no tags present
Revert "Added branch-alias for dev-master to ```--version|-V``` output when current", fixes #3356

This reverts commit 225a8f7a5660e12ad53298859a6674293cca3cb9.
Added branch-alias for dev-master to --version|-V output when current version is actually a revision (a non release/git tag).
This replies the work done in #3352 respecting other uses of ```Composer::VERSION``` constant.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| Doc PR        | none