Implement 'indent' filter

Implement 'indent' filter according to the [jinja2 specification](https://jinja.palletsprojects.com/en/2.10.x/templates/#indent):

> Return a copy of the string with each line indented by 4 spaces. The first line and blank lines are not indented by default.
>
> **Parameters:**
> - width – Number of spaces to indent by.
> - first – Don’t skip indenting the first line.
> - blank – Don’t skip indenting empty lines.
>