Add the ability to make it so tags can be added to all datapoints from t...

...he CLI/conf. Fixes #153

A few things I noticed that concern me:
 - opentsdb.TagSet.Merge is destructive but also returns the tagset. That threw me off, is this normal?
 - Setting the hostname and and now these additional tags is duplicated in both program.go and AddTS, should this be refactored now?
dpTags = AddTags.Copy().Merge(dp.Tags)