Added command line utility to render templates; partials can be passed as a dictionary

[vrde]

Template __init__ now accepts the dictionary partials.
If partials is defined, templates will be loaded using the dictionary instead of the regular "file loader".
Those changes should be retro compatible.

I also added a command line utility to test/render templates.

Synopsis

usage: pystache [-h] template context

Render a mustache template with the given context.

positional arguments:
  template    A filename or a template code.
  context     A filename or a JSON string

optional arguments:
  -h, --help  show this help message and exit

[cjerdonek]

I'm going to create a second issue for passing partials as a dictionary so that it can be treated separately from the command-line utility part.