Add option to indicate search range is not begin/end of string

This mimics `ONIG_OPTION_NOTBOL`/`NOTEOL` but works for `\A` and `\z` instead of `^` and `$`.

The tests I used while adding the option: https://gist.github.com/sorbits/6261329 (it’s unclear to me how the ruby test file is supposed to be used).

If you accept this pull request then remember to translate description in `doc/API.ja`.