Allow env_options in `Jinja2Templates

[aminalaee]

Closes #427

Allows Jinja2Templates to set Jinja2 Environment options here.

For example:

templates = Jinja2Templates(
        directory=directory, autoescape=False, auto_reload=True, enable_async=True
)

Or:

loader = jinja2.FileSystemLoader(directory)

templates = Jinja2Templates(
        directory=directory, loader=loader
)

Since directory is still a required argument, it will be passed in this case, but the loader passed will take precedence over the directory and the loader created by Starlette. I can't think of any way to handle this right now, until maybe we can consider another API for Jinja2Templates later. This way we can deal with issues like #1214 .

• Tests
• Docs

[aminalaee]

We can decide if we want to allow loader to be passed in env_options or not. The point is if we add it it makes directory reduntant but allows user full control over the loader.

[tomchristie]

Neat. 👍

[aminalaee]

The documentation was not done yet. Can you check again?

[tomchristie]

Yup - I oughta have noted that. 😅
Docs look good.