Add prev/next post link to current post

Signed-off-by: Chengwei Yang <chengwei.yang.cn@gmail.com>
limit excerpt on home page to 50 words

Signed-off-by: Chengwei Yang <chengwei.yang.cn@gmail.com>
Reuse postnavigation class to create older/newer post

Signed-off-by: Chengwei Yang <chengwei.yang.cn@gmail.com>