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.

The configuration example of https://github.com/tomohisaota/fluent-plugin-tail-multiline#case-where-first-line-does-not-have-any-name-capture case:

```aconf
<source>
  type tail
  path /path/to/tail/file
  tag tail.multiline

  format multiline
  format_firstline /----/
  format1 /time=(?<time>\d{4}-\d{1,2}-\d{1,2} \d{1,2}:\d{1,2}:\d{1,2}).*message=(?<message>.*)/
</source>
```
Need feedback:

- `format_firstline` name. Is there more better parameter name?
- `none` support is needed?

@tomohisaota Could you check this changes?