Add better control for what post types are available in the plugin

[Anderson Grüdtner Martins]

Right now we only show post types that follow the criteria:

public = true
+
public = false, show_ui = true, _builtin = true

Some users want to use non-public post types.
Maybe we could add an option to easily select what post types should be displayed by the plugin, or a filter that allows to customize the criteria or the list of post types.

Related to #377