[6.x] Allow commands to overwrite their input and output handlers

[gocanto]

This [PR] allows commands to overwrite the input and output handlers.

This is useful when you need to swap out a current handler on runtime execution. It also adds an easy way to test commands as Units without the need to extend from the framework test case class to write tests as it is in here.

There is another way to hook into this cycle by mocking the Laravel value, but you need to get into setting all the expectations needed as shown in this Test

[GrahamCampbell]

Please use fully qualified class names in the phpdoc, and put 2 spaces around the type in param docs.

[gocanto]

Sure thing Graham. I will get to it tomorrow!

[gocanto]

@GrahamCampbell I think these commits should do the trick. Thanks!

99746e8

6de2286