[bw_new] - support updates to existing content

[bobbingwide]

For bigrams I want to be able to update existing entries if one already exists but doesn't have an attached featured image.
See bobbingwide/bigram#28

Requirement

• Ability to update an existing post with the given title
• Ability for plugins to determine the post's new field values

Solution

• Extend the [bw_new] shortcode to check if the post already exists and offer an update capability.
• Implement a filter called bw_new_pre_update_post to enable a plugin to indicate whether or not it's happy for the update to proceed and for the plugin to set the new value for post_content, etc.
• It's the plugin's responsibility to applying the changes to the post's fields.
• If the filter returns null then don't update the existing post.
• The logic will only apply for logged in users who can "publish" a new post immediately. ie when the post_status is "publish".