added fn to write doc to output stream

This new function is essentially a convenience wrapper which leverages (but hides) the existing temp-file based approach. Tested successfully using the following ring handler.

    (ns foozle.core                                                                                 
      (:require [excel-templates.build :as excel]                                                   
                [ring.util.io          :as io]))                                                    
                                                                                                    
    (defn handler                                                                                   
      [req]                                                                                         
      {:status 200                                                                                  
       :headers {"content-type"                                                                     
                 "application/excel"                                                                
                                                                                                    
                 "content-disposition"                                                              
                 "attachment; filename=foo.xlsx"}                                                   
       :body                                                                                        
       (io/piped-input-stream (fn [output-stream]                                                   
                                (excel/render-document-to-stream!                                   
                                  "foo.xlsx"                                                        
                                  output-stream                                                     
                                  {"Sheet1" {0 [[1 1 1 1 1]]}                                       
                                   "Sheet2" {0 [[2 2 2 2 2]]}                                       
                                   "Sheet3" {0 [[3 3 3 3 3]]}})))}) 
Why the ! on the function name? Because it modifies the stream?

What do you think about modifying both the function names to `render-to-file` and `render-to-stream`? That gives us a nice relationship between the function names and their targets.

I'm not too concerned with backwards compatibility yet, since the library is only used in a couple of places so far.