Move command over to new v2 structure

[wojsmol]

fixes #20
test results from my fork https://travis-ci.com/wojsmol/wp-super-cache-cli.

[wojsmol]

fixes #20

[szepeviktor]

fixes #20

It needs to be added in the top comment to actually close #20.

[wojsmol]

@szepeviktor Ives if github don't catch this then this will be reference for @schlessera 😄

[schlessera]

This is a testing requirement, it should be moved into "require-dev" section.

[schlessera]

Ah, clicked on the request review button too fast...

@wojsmol Please make the following additional changes:

• move cli.php into src/WPSuperCache_Command.php
• add PSR-4 autoloader that maps src/ to an empty namespace
• add a wp-cli.yml file that requires the wp-super-cache-cli.php file

[wojsmol]

I'm temporally using in tests this link /~https://github.com/wojsmol/wp-super-cache-cli/archive/tov2.zip

[wojsmol]

Thank you @swissspidy 😄