title: numpy.savetxt format specifier for array
body: <p>I'm using <code>numpy.savetxt</code> to save my matrix of <code>dtype=[('time','S8'),('channels', 'float16', 2)]</code>
  A row of this matrix looks like:</p>
  
  <pre><code>(b'12:31:24', [ 1.20019531,  4.        ])
  </code></pre>
  
  <p>Is there any way to specify this format? I was trying to use <code>fmt='%s %.3f %.3f'</code> but it's not working. <code>numpy.savetxt</code> requires pythonic format specifier, not numpy dtype and I can't figure out how to specify array.</p>
  
  <p>If I use different <code>dtype=[('time','S8'),('channel0', 'float16'),('channel1', 'float16')]</code> everything works fine, since I get rid of this array and get only columns with floats but I would like to use the dtype with one string and array per row (there are more channels in my device and having data stored in an array simplifies iterating)</p>