utf8: add function to align a string into given strbuf

utf8: add function to align a string into given strbuf

Add strbuf_utf8_align() which will align a given string into a strbuf
as per given align_type and width. If the width is greater than the
string length then no alignment is performed.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>