Add ability to trim at number of paragraphs

A client requested to make smart_trim work with number of paragraphs instead of characters or words to have a neat output (without syllables for example). The client is using smart_trim because core's read more is always at the end of the complete teaser and not right behind the body text. So I made a simple patch to allow users to break text fields after a set number of paragraphs.
I am not sure if this functionality is already somehow available through core's text.module, so this patch may be redundant.

Please note: This patch was made for a system using CKEditor module and library. The patch only checks for paragraph tags <p>, leaving out everything outside these tags. Also, there's currently no fallback if there's no paragraph tag at all.