feature: implement -S --stream option

[Kamilcuk]

Implement -S --stream option for taking input from stdin and outputting to stdout.

$ jinja2 -S <<<'{% set a = 1 %}{{ a }}'
1

Thanks!

Solves https://github.com/mattrobenolt/jinja2-cli/issues/129 .

Please kindly also look at https://github.com/mattrobenolt/jinja2-cli/pull/126 .