Fixes stdin redirection issues within the command-line module.

The callback passed into `command-line.run` would return the argument list:
`stderr, stdout`, however, these were not populated in the command-line module
as `stdio` was set to 'inherit' in the `spawn` function call.

This meant that the `error` argument of the callback would always be a falsey
empty string.  The result of this was that the sass gem was always downloaded
and installed each time `origami install`. The `stdout` stream from
`sass --version` was being sent directly to the real 'stdout'.

This alters the API providing a single point for spawning processes which adds
an optional `option` argument for controlling stderr and stdout.
Ensure 'stdio: inherit' so that errors are displayed from the SASS compiler during the build output.  It seems that individually setting inherits on each stdio member is ineffective.
Make argument naming consistent with the other callback passed to the same function in the file.
Move the callback to the final parameter and alter function invocations to reflect this change.
Merge pull request #30 from samgiles/command-line-fixes

Fixes stdin redirection issues within the command-line module.