Add new separate filter_module and filter_level to Builder

[sebasmagri]

No description provided.

[KodrAus]

I think let's do this. The idea is we add two new methods to the fmt::Builder and root Builder types:

fn filter_module(&mut self, module: &str, level: LevelFilter) -> &mut Self {
    self.filter(Some(module), level)
}

fn filter_level(&mut self, level: LevelFilter) -> &mut Self {
    self.filter(None, level)
}

I think it's worthwhile because filter(None, level) actually means apply to logs from all modules. That seems a bit odd, since we're passing a value of None to the module.