Add the "product" and "product/wordpress" structure

[terrorbyte]

Currently go-exploit does not have any public product specific handling, but I've had to write a few WordPress plugin RCEs lately and was getting tired of reinventing the wheel.

This lays the groundwork to have product specific functions and handling. This also only implements: authentication, nonce selection, and plugin upload based RCE.

In the future I had a few thoughts that we could potentially add:

• flag handling for each product specific implementation allowing a wordpress.Flags() call so that the CLI flags can automatically inherit any product specific flags in the future.
• Offer per-product specific selection of payloads (related to above) so that you could potentially modify the exploitation path further.

[j-baines]

Looks good to me, I just had the two basic comments (use framework logging and possibly move the one function). Other than that, looks good.