Add `aggregate`, `add_tag_prefix`, `targets`, `suppress_tick`, and more options

This is a huge commit to add following options. 

* `aggregate tag` which enables to watch multiple tags seperately
* `add_tag_prefix` and `remove_tag_prefix`
* `targets` which enables to watch multiple fields, and `thresholds` to set thresholds for each field. 
* `suppress_tick` which suppress to emit output messsages during specified seconds after starting up.

And, I changed README.rdoc to README.md.