add replace_tag_separator option

Hi.
I want a placeholder replaced with underscored tag (replaced '.' with '_').

Some storages (ex. mysql) are not support the tag separated by '.' .
So some configure can not use forest like below:
```
<match access.foo.example.com>
  type mysql
  table foo_example_com
</match>
<match access.bar.example.com>
  type mysql
  table bar_example_com
</match>
```

I have two idea for this issue.

1. make configure attribute named `replace_tag_separator`
2. make a new placeholder like `${tag.replace_separator}`

I implemented the first idea in this PR.
This patch allows to write the configure like below:
```
<match access.*>
  replace_tag_separator true # replace '.' with '_'
  replace_tag_separator +    # replace '.' with specified separator '+' 

  <template>
    table ${tag}
  </template>
</match>
```