Add new follow method

[maGitty]

First, thanks a lot for providing this interface to systemd, I currently use it to synchronise the journal of multiple devices with an external remote.

The recent follow method didn't work as I wanted it for my use case to due to various reasons:

• After each wait, only one one journal entry was written to the writer
  • So, if more than one entry adds to the journal within the specified 100ms, the journal stays behind the tail
• The formatter hook is quite extensible, but channels might give slightly higher flexibility
• Skipping entries manually from outside is not possible

[maGitty]

Thanks a lot for your suggestions @lucab! I implemented some of them already and will come back for the rest soon. Especially enabling better asynchronous behaviour of the FollowTail method will surely increase the number of possible use cases.