Add interface

[psvetlov]

fixes #856

[andersjonsson]

I definitely see the use case for this.

What do you think about making the interface accept a PathString and return another PathString? The current design was already established, but if we want to encourage people to implement their own tuners we might want to make it easier to use for it's intended purpose? What do you think? Or do you think one would want to check headers and other stuff?

[andersjonsson]

Thanks again for this contribution. I'll give this another review as soon as you've had the time to fix the unit test and address the design issues we discussed