* Use UTF-8 for multi-byte character I/O everywhare

This fix is required for "libstdc++".
Without this fix, "libstdc++" only works for 7bit ASCII.

It fixes https://github.com/microsoft/pict/issues/24 .

* Use current locale instead of fixed UTF-8 locale

It is good for non-UTF-8 (ANSI/OEM) encoding input.
Use environment variable "LANG" or "LC_CTYPE" to change input encoding.

(example)
$ LC_CTYPE="C.UTF-8" pict utf8-input.txt

* Add document for non-ASCII (UTF-8/ANSI) input encoding