Introduce wp_make_excerpt

[filosofo]

Currently, there is no sensible way to generate an excerpt, given a text string of text.

If you pass a non-empty string of text to wp_trim_excerpt, it just returns the text to you.

Patch introduces wp_make_excerpt, which given a string and optional length and ellipses parameters, returns that string, excerpted.

[nacin]

#16372

[sivel]

See #12858 also. #16372 and #12858 look similar if not the same, might need to close one of those also.