Improvements to the CLI commands

[renatonascalves]

• Improving PHPDoc
• More CLI command examples
• Adding more standards from https://make.wordpress.org/cli/handbook/guides/commands-cookbook/
• Using WP_CLI::log instead of WP_CLI::line
• The debug command checks for the existence of a valid post and adds a warning for SP_DEBUG and SAVEQUERIES
• Use WP_CLI\Utils\get_flag_value to get flag values
• Use quote instead of double quotes