Make more use of SPI

[zcourts]

Taking advantage of Java's service provider interface is now a better choice having found the SPI project (cloned here https://github.com/zcourts/spi).

The SPI project means this is now less error prone. With that in mind, as the project seems to be expanding, a lot of it can be extracted into self contained modules. The recent experiment to support multiple HTML templating languages is a good example.
See a9a06d1 ...