Broadcast writing on README no longer compiles (due to removal of `broadcast()`)

The "broadcast writing (simple file copying)" example on the [README.md](https://github.com/a8m/pb/blob/1747edaf2fc839b9542a0476c070ee6fb3effa07/README.md) no longer compiles. This is because `Writer::broadcast()` was never stabilized and [removed from Nightly libstd on Mar 8, 2016](https://github.com/rust-lang/rust/commit/b53764c73bd722ea22142bace6249d5950066253#diff-668f8f358d4a93474b396dcb3727399eL1128).

I think a simple workaround is to change the example as the followings:

```rust
extern crate pbr;

use std::io::copy;
use pbr::{ProgressBar, Units};
use std::fs::File;
use std::io::Write;

fn main() {
let mut file = File::open("/usr/share/dict/words").unwrap();
let n_bytes = file.metadata().unwrap().len() as usize;
let mut pb = ProgressBar::new(n_bytes as u64);
pb.set_units(Units::Bytes);
let mut handle = File::create("copy-words").unwrap();
{
let mut tee = TeeWriter::new(&mut handle, &mut pb);
copy(&mut file, &mut tee).unwrap();
}
pb.finish_print("done");
}

/// Tee (T-Split) Writer writes the same data to two child writers.
struct TeeWriter<'a, T1: Write + 'a, T2: Write + 'a> {
w1: &'a mut T1,
w2: &'a mut T2,
}

impl<'a, T1, T2> TeeWriter<'a, T1, T2>
where
T1: Write + 'a,
T2: Write + 'a,
{
fn new(w1: &'a mut T1, w2: &'a mut T2) -> Self {
Self { w1, w2 }
}
}

impl<'a, T1, T2> Write for TeeWriter<'a, T1, T2>
where
T1: Write + 'a,
T2: Write + 'a,
{
#[inline]
fn write(&mut self, buf: &[u8]) -> std::io::Result {
let size1 = self.w1.write(buf)?;
let size2 = self.w2.write(buf)?;
Ok(size1.min(size2))
}

#[inline]
fn flush(&mut self) -> std::io::Result<()> {
self.w1.flush()?;
self.w2.flush()
}
}
```

Or, for convenience, maybe we can add something like `TeeWriter` above to this crate.

I could work on either ideas. Please let me know your thoughts.