Doesn't support engines that are installed from scoped packages

I have a private engine at `@my-private-scope/aglio-theme-foo`. When I do `aglio -t foo`, aglio is unable to find this module, because it's only looking for modules like [this](https://github.com/danielgtaylor/aglio/blob/master/src/main.coffee#L55):

```javascipt
require('aglio-theme-' + theme)
```

where `theme` is `foo` in my case.

It would be nice to be able to take a whole module path as the template engine like:

```bash
aglio -t @my-private-scope/aglio-theme-foo
```

thanks

Start in src/main.coffee at the module lookup near line 55, then run the documented `aglio -t @my-private-scope/aglio-theme-foo` command with a scoped engine. Done means Aglio accepts a full scoped module path as the template engine while preserving the existing short `foo` form.

Written by the indexing model from the issue text.