added 'multiple' option to command line render

Please merge this request. It adds a "--multiple" option to command line render of pystache that renders the provided template for each of the context children.  
Output is saved in a file named per either a key in context, or a user-given string.

Usage (in pystache/pystache/tests/examples): 
`pystache multiple.mustache multiple.json -m out`    (to generate 3 files out.000, out.001, out.002; if there is no 'out' key in multiple.json)
`pystache multiple.mustache multiple.json -m NO`    (to generate 3 files names from the value of 'NO' key in multiple.json)