Marker \ipc for centered paragraphs in introductory material

[PaulusKieviet]

I usually mark additional material (including books like FRT and INT) with introductory codes like \ip, \im and \ib. In this introductory material, itꞌs common to have centered paragraphs, e.g. for the publisherꞌs name on the title page, or various information on the verso page. It would help consistency if a marker \ipc would be available to use in addition to \ip, \im etc.

[mhosken]

Added 3.1.1