Implement 'indent' filter

[flexferrum]

Implement 'indent' filter according to the jinja2 specification:

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.