Merge in_tail_multiline into in_tail. refs #269

First step of https://github.com/fluent/fluentd/issues/269, merging in_tail_multiline feature.
The different from original in_tail_multiline is below:

- 'format multiline' is required.
- Use `format1 /multiline regexp/` instead of `format /multiline regexp/`.
- No automatic flush. At shutdown phase, remaining lines are flushed.

Need feedback:

- `format_firstline` name. Is there more better parameter name?
- `none` support is needed?

@tomohisaota Could you check this changes?