added new command for changing property settings

As of #1411 I have added a new Command for changing simple properties like
- name
- description
- homepage
- stability
- license

Example calls:
```
composer property-set name "foo/bar"
composer property-set stability "beta"
```

`JsonFile` is internally used for rewriting the json file so older files might get restructured with this.


Things to discuss:
- I re-used some logic from `RequireCommand` and moved it to `Command\Helper\FileHelper`: Should be `RequireCommand` adapted accordingly?
- I tried to test several tests for this new command. Currently not many test cases for commands are existing and so I want to go sure if the used workflow is OK for you