Custom format templates

I rather like the flexibility of the -template option in tj, though there it seems to come at the cost of performance (~5MB/s - compared with rtss's 220MB/s) and line buffering.

I have a quick and dirty prototype that manages a 90% solution while still running at 70MB/s, with obvious opportunities for caching - I dare say using a proper Go template crate will be considerably slower, but I probably don't need that flexibility.

This will provide a nice general solution to #2 , while providing opportunities for other nice stuff like output colouring.