Add --autoload option to 'wp option add' command

Closes #1113

I don't know what is the best way to create a test for this new option. There is no straightforward way to get the value of the autoload field using wp-cli.

One hackish possibility is to use `wp db query` to query wp_options directly and check if the autoload field was populated correctly. Should I create a test using this approach or should I leave this part untested? Do you see another option to test this?