sub: add justify of subtitles

To make it easier for the eyes, multi line subtitles should
be left justified (for most languages).
This adds an option to define how subtitles are to be justified
inpendently of how they are aligned.
Also add option to enable --sub-justify to be applied on ASS subtitles.

sub/osd.c

@@ -62,6 +62,8 @@ static const m_option_t style_opts[] = {
     OPT_FLOATRANGE("blur", blur, 0, 0, 20),
     OPT_FLAG("bold", bold, 0),
     OPT_FLAG("italic", italic, 0),
+    OPT_CHOICE("justify", justify, 0,
+               ({"auto", 0}, {"left", 1}, {"center", 2}, {"right", 3})),
     {0}
 };