Ability to do dry run instead of actual migration generation (cycle:migrate)

[alexndr-novikov]

It will be useful for developers to have an ability to see changes before actual generation of migration file.

Usual workflow of development that includes schema changes now looks like this:

1. cycle:migrate
2. wait till file appear in host system filesystem (when using docker volume it takes 1-2 seconds to appear in file system and be indexed by phpstorm)
3. open file, check diff.
4. if something is missing - remove file, make entity file schema changes, go to p.1

My suggestion is to add some flag like --dry, which will not generate actual file, but just print the result to console, where you can immediately see the difference

php app.php cycle:migrate --dry