Multi-byte Unicode support

When using “truncate at n characters” option, if the last character before the ellipses is a Unicode character, then it sometimes displays incorrectly (question mark, box with question mark, etc). An easy fix is possible simply by changing “$t = substr( $t, 0, $c );” line in class/advanced-excerpt.php to “$t = mb_substr( $t, 0, $c );”. The latter function mb_substr handles multi-byte characters correctly. Please consider implementing this fix.