Multi line text output for the font addon.

Add al_draw_multiline_text, al_draw_multiline_textf, and
al_draw_multiline_ustr.  Text will be broken into lines either on
a newline character '\n' (but not carriage return '\r'), to force
a line break, or on a space or tab if the text is wider than
the max_width parameter. The line height can be set as a parameter
or when set to 0, the font's line height is used as a default.
There are also al_do_multiline_ustr and al_do_multiline_text
for advanced text layout needs.  There is an example program
ex_font_multiline.cpp to test out these functions with.
De-duplicate the multiline-text documentation
Make ex_font_multiline more snazzy