Fixes #5610 - Allow `--generate` option for gii cli and overwrite existing files when --generate=1 and --interactive=0

Fixes #5610 - Gii with --generate=1 is not available in cli and does not overwrite existing files as is claimed in the phpdoc

Testcase:
1. Set-up the basic app as instruction on https://github.com/yiisoft/yii2/blob/master/docs/internals/getting-started.md#getting-started-with-yii2-development
2. Add an empty database and adjust db credentials
3. Run the following commands:
```
$ ./yii migrate
Yii Migration Tool (based on Yii v2.0.1-dev)

Creating migration history table "migration"...done.
No new migration found. Your system is up-to-date.

$ ./yii gii/model --tableName=*
Running 'Model Generator'...

The following files will be generated:
        [new] models/Migration.php

Ready to generate the selected files? (yes|no) [yes]:

Files were generated successfully!
Generating code using template "/local/path/to/yii2-pr-base/extensions/gii/generators/model/default"...
 generated models/Migration.php
done!

$ ./yii gii/model --tableName=*
Running 'Model Generator'...

The following files will be generated:
  [unchanged] models/Migration.php

No files were chosen to be generated.

$ echo "a-change" >> models/Migration.php

$ ./yii gii/model --tableName=*
Running 'Model Generator'...

The following files will be generated:
    [changed] models/Migration.php
Do you want to overwrite this file? [y,n,ya,na,?]: ^C

$ ./yii gii/model --tableName=* --interactive=0
Running 'Model Generator'...

The following files will be generated:
    [changed] models/Migration.php

No files were chosen to be generated.

```

Without any of these commits, the following happens:
```
$ ./yii gii/model --tableName=* --interactive=0 --generate=1
Error: Unknown option: --generate
```

With [the first commit](https://github.com/motin/yii2/commit/80f4e46f5579df3aa94e67030f304d63fd31e10d) but without [the second commit](https://github.com/motin/yii2/commit/c7c855fc3321b2e9cdbd3d547b9bb1008748b756), the following happens:
```
$ ./yii gii/model --tableName=* --interactive=0 --generate=1
Running 'Model Generator'...

The following files will be generated:
    [changed] models/Migration.php

No files were chosen to be generated.
```

With the second commit, the desired behavior is achieved:
```

$ ./yii gii/model --tableName=* --interactive=0 --generate=1
Running 'Model Generator'...

The following files will be generated:
    [changed] models/Migration.php

Files were generated successfully!
Generating code using template "/local/path/to/yii2-pr-base/extensions/gii/generators/model/default"...
 overwrote models/Migration.php
done!
```

The same procedure for crud generation:

```
$ ./yii gii/crud --interactive=0 --generate=1 --modelClass=\\app\\models\\Migration --controllerClass=\\app\\controllers\\MigrationController
Running 'CRUD Generator'...

The following files will be generated:
        [new] controllers/MigrationController.php
        [new] views/migration/_form.php
        [new] views/migration/create.php
        [new] views/migration/index.php
        [new] views/migration/update.php
        [new] views/migration/view.php

Files were generated successfully!
Generating code using template "/local/path/to/yii2-pr-base/extensions/gii/generators/crud/default"...
 generated controllers/MigrationController.php
 generated views/migration/_form.php
 generated views/migration/create.php
 generated views/migration/index.php
 generated views/migration/update.php
 generated views/migration/view.php
done!

$ echo "a-change" >> controllers/MigrationController.php

$ ./yii gii/crud --interactive=0 --generate=1 --modelClass=\\app\\models\\Migration --controllerClass=\\app\\controllers\\MigrationController
Running 'CRUD Generator'...

The following files will be generated:
    [changed] controllers/MigrationController.php
  [unchanged] views/migration/_form.php
  [unchanged] views/migration/create.php
  [unchanged] views/migration/index.php
  [unchanged] views/migration/update.php
  [unchanged] views/migration/view.php

Files were generated successfully!
Generating code using template "/local/path/to/yii2-pr-base/extensions/gii/generators/crud/default"...
 overwrote controllers/MigrationController.php
   skipped views/migration/_form.php
   skipped views/migration/create.php
   skipped views/migration/index.php
   skipped views/migration/update.php
   skipped views/migration/view.php
done!

```