Add Tera v2 support

[axoking]

I added a feature tera2 which will use tera v2 instead of the deprecated v1. I left the old tera feature working for compatibility.

An if condition that checks if the template exists could be omitted, because tera 2 early returns before that function is even called if the template does not exist.

I did not write any documentation or testing, because I'm not familiar with that. But I tested both tera and tera2 with a seperate crate I wrote, and they work and render using the correct tera version.