Enable caching of compiled Jinja templates

[oprypin]

This should reduce startup times a bit.

[ultrabug]

This idea is cool and I already feel like it could benefit multi-language builds such as when used with the mkdocs-static-i18n tho I'd have to check.

While the FileSystemBytecodeCache implements a clear() method but it is not called by default which means that we leave potentially sensible templating information in the /tmp directory of users after a build.

I'm unsure about where we should trigger a clear() to still allow users to benefit from caching but I'm under the impression that we should allow users to opt in or out this feature as well.

[oprypin]

The only point of this cache is to persist across invocations. If it is cleaned up on exit, that's the same as not having it

[ultrabug]

My point is that multi-build users could benefit from this while still clearing the cache at the very end (if we make it possible / tunable)

[oprypin]

True, but that can also be achieved without writing anything to disk and instead just preserving the jinja2.Environment object.
Yeah worth thinking