Add flush_tables

[Sable-20]

Flush tables via iptables -F command without requiring specific tables.

A more drastic measure that can be taken instead of adding a function can be used:

pub fn flush_table(&self, table: Option<&str>) -> Result<(), Box<dyn Error>> {
    if table.is_none() {
        return self.run(&["-F"]).and_then(output_to_result);
    }
    self.run(&["-t", table, "-F"]).and_then(output_to_result)
}

While a breaking change this would require people to memorize one less function, this would also be a fairly trivial fix for people to wrap table in their calls to flush_table with Some().

// new call
ipt.flush_table(Some("table"));