Tag mapped index name

Hi, I'm using this plugin to watch logs on Kibana and very helpful for me.

In my use case, I want to send multiple kind of logs to separated indices.
So I want to change target index by received tag name dynamically.

I added two options for it.
If `tag_mapped` is true, index name would be changed to received tag name. This also works with `logstash_format`.
And if `remove_tag_prefix` is set, specified prefix string is removed from tag.
Please look README and test cases for detail.