Create `dustr` tool to render templates on the command line

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Replaces #354 since we removed the old `dustc` tool.

We will create a command-line tool that accepts a template and context, and renders the result to stdout or a file.

Use case: Generating static sites using Dust as a generator. Unit testing your app that uses Dust.

API examples:

```
$ dustr foo.dust --context=context.js
$ dustr tmpl/foo.dust --context=context.js --pwd=tmpl --output=site/foo.html
$ dustr tmpl/*.dust --output=site # split is automatic with dustr, creates site/foo.html, site/bar.html
$ dustr foo.dust --whitespace=true # no context required
```

`dustr` will implement an `onLoad` function to load and compile partials in the same way that `dustc` compiles them.

Start by reviewing the removed dustc tool and issue #354 to understand the existing compilation behavior, then trace how Dust templates and partials are currently loaded. Done means a dustr command accepts the described template, context, whitespace, working-directory, and output options, renders to stdout or files, expands globs, and loads partials through onLoad.

Written by the indexing model from the issue text.