Adding help for custom importers

Currently as the importers are decoupled from the main wp-cli command, it does not have a way to show the arguments for each importer via the `wp help` command.

During discussion ( with @johnbillion and @tcrsavage ), we came up with couple of ways to fix this:
1. Importers can extend the main importer and become a second level command and thus be able to define their arguments within `wp help`.
2. Can add a dedicated method that generates/output the documentation for individual importers within the main wp-cli command.