* Updating `batslib_count_lines` to automatically detect trailing newlines and count them if present.
  * This works on the concept that a string has a trailing newline if it was intended -- e.g. via `run --keep-empty-lines` or similar.
* Updating `batslib_prefix` and `batslib_mark` to accept `--keep-empty-lines` option to properly print empty lines and trailing newlines.
* Updating `batslib_print_kv_single_or_multi` calls to `batslib_prefix`.
  * Will automatically detect trailing new lines (on a per-value basis).
  * If trailing newline is present, `--keep-empty-lines` will be passed to `batslib_prefix`.

This implements option 4 for `batslib_count_lines` and `batslib_print_kv_single_or_multi`, and option 2 for `batslib_prefix` and `batslib_mark`, as discussed in bats-core#11.

Fixes bats-core#11.