Add p2p plugin for static analysis

[szepeviktor]

Adding p2p:

composer require --dev scribu/lib-posts-to-posts:dev-master scribu/scb-framework:dev-master

Analysis result:

 ------ -----------------------------------------------------------
  Line   src/PostTypeAdmin.php
 ------ -----------------------------------------------------------
  1242   Cannot call method each_connected() on object|true.
  1474   Cannot call method direction_from_types() on object|true.
  1475   Cannot access property $side on object|true.
 ------ -----------------------------------------------------------

@johnbillion I leave this 💯% up to you.

[szepeviktor]

The first step could be to add strict conditions.

// if ( $ctype )
if ( is_object( $ctype ) )
// or
if ( $ctype instanceof \P2P_Connection_Type )

But I do not know how this project operates.