[RFC] Add target option to all commands (dir relative to the project root)

Related with https://github.com/symfony/maker-bundle/issues/2 and https://github.com/symfony/maker-bundle/issues/49

What about a new target option for each command that allows us to change the target directory where the final skeleton file is generated?

For example make:unit-test FooTest always creates the file class inside tests/ directory, but as the best practice recommends:

By convention, the tests/ directory should replicate the directory of your bundle for unit tests. So, if you're testing a class in the src/Util/ directory, put the test in the tests/Util/ directory.

Something like this make:functional-test FooTest --target=tests/Util will be great to not move each generated file to the corresponding directory and change its namespace.

Start by reviewing the make:unit-test and make:functional-test command entry points, then compare how the other commands choose their generated-file directories. Check the related issues #2 and #49 for context. Done means every command consistently accepts a target directory relative to the project root and generates the file there with the matching namespace.