Create composer replace section

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | [yes in test project](https://travis-ci.org/patkar/SyliusReplaceTest/builds/25591085)
| Fixed tickets | #1503
| License       | MIT
| Doc PR        | n/a

Works like a charm :shipit: