Set b:sy_path also for SessionLoadPost events

Closes #101.
Fix unicode + subscripting bug

This bug happened when a unicode character was used for the delete sign.
Prior to this patch we used string subscripting which worked fine for
ASCII characters, but see yourself:

    :echo '«5'[-2:]
Let counter overflow if more than 100 lines deleted

If there are more than 99 deleted lines, the actual count isn't being
important anymore. Thus we just let it overflow and save another
conditional.

100 deleted lines -> "00"

Closes #109.
Doc: correct delete signs explanation
Restore behavior of using _> for SignifyDeleteMore signs

Regression was introduced by 7f57ee5a.

Signed-off-by: James McCoy <vega.james@gmail.com>