feat: add `Tera:new_uninitialized` (#677)

[seancroach]

Exposes a new function that creates an instance of Tera without registering the default filters, testers, and functions. If this is merged, the version of tera in Cargo.toml will have to get its minor version incremented. No tests were added as no new code was created (except for a function declaration); I simply cut and paste the body from the Default implementation. I need this functionality and think no-ops as an overwrite are too hacky. I hope others can benefit from this, too.

[Keats]

I'm not a fan of the name but I can see the need hmm

[seancroach]

That's totally fine. I made this rather quickly, and just made it... literal. Augment it however you'd like; I have no strong opinions on the matter.