Fixes this failure:

      1) Mustache CLI without partials writes rendered template into the file specified by the third argument:
         Uncaught TypeError [ERR_INVALID_CALLBACK]: Callback must be a function
          at makeCallback (fs.js:136:11)
          at Object.unlink (fs.js:943:14)
          at test/cli-test.js:86:12
          at ChildProcess.exithandler (child_process.js:285:7)
          at maybeClose (internal/child_process.js:982:16)
          at Process.ChildProcess._handle.onexit (internal/child_process.js:259:5)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

* Fixes CLI tests for windows

* Hijacked exec so we don't have to wrap each command in changeForOS, and made replacing 'cat' safer with word boundaries

* Fixing changed tests to be more consistent with the code base

Fixes #431

CLI used to render a mustache template with a data view, writes the template into stdout when successfull. Otherwise meaningfull errors into stderr.

Fixes #424