Add ability to grab custom block from name.

This allows developers to access custom blocks from other plugins without
the need for dependenices. You only need the plugin's name and the custom
block's name.

Signed-off-by: Chris Sanders <zidane@outlook.com>
Add ability to grab custom item by name.

Same as with the CustomBlock commit, simply pass pluginname.item or
separate (preferred).
Signed-off-by: Chris Sanders <zidane@outlook.com>
Fix NPE when looking up item/block.

Signed-off-by: Chris Sanders <zidane@outlook.com>
Didn't mean to push that...
Signed-off-by: Chris Sanders <zidane@outlook.com>