Consider publishing tailer separately

Hi,

I'm currently using a chunk of mtail as a replacement for [hpcloud/tail](https://github.com/hpcloud/tail). I was previously pulling google/mtail/[tailer,watcher,logline], but with the latest changes on master this isn't possible anymore. I agree with the move to `internal` and I can always maintain my own copy and make updates but it would be very useful to have the tail implementation in mtail exposed as a standalone library. It's outside of the scope of mtail to expose a library, but maybe useful to the golang community as a whole :)