Added options for instructions at the top and bottom of custom meta boxe...

[theoephraim]

Lets you add some content at the top and bottom of custom meta boxes by setting
instructions_pre and instructions_post properties on any add_meta_box calls.

Had to change how arguments are passed around to the meta box callback slightly to make it work.

[mboynes]

Hi @theoephraim, sorry for the embarrassing delay in responding, and thanks for the PR! I like what you've done here. I might use the core of your code but merge it into one field for simplicity. I'll reference this ticket when I commit that, and give you a shout out. I'm going to leave this open until it gets merged in. Thanks again!