Move data-liberation WP-CLI command to separate class

[zaerl]

Move the wp data-liberation a-file.xml WP-CLI command to a separate class.

It should ideally be changed this way:

1. A first subcommand import that is callable as wp data-liberation import a-file.xml. So that we are open to other subcommands. E.g. wp data-liberation analyze, preview, restart etc.
2. A --dry-run option.
3. pcntl_signal for catching SIGINT and SIGTERM that call exit() when the user presses ctrl+c on the keyboard.
4. Should output helpful information to the user during the various phases of WP_Stream_Importer::next_step().