Function ftml::includes::include

pub fn include<'t, I, E, F>(
    log: &Logger, 
    input: &'t str, 
    includer: I, 
    invalid_return: F
) -> Result<(String, Vec<PageRef<'t>>), E> where
    I: Includer<'t, Error = E>,
    F: FnOnce() -> E,