Customize HTML templates

Customize html templates in strong-error-handler, see https://github.com/strongloop/loopback/issues/1650#issuecomment-161920555 for details.

Configuration options:

``` js
// map of templates to use for rendering responses
views: {
// the default view
default: path.resolve(__dirname, 'views/error-default.jade'),
404: path.resolve(__dirname, 'views/error-not-found.jade'),
// etc.
},

// map of static files to send as a response
files: {
// default: path.resolve(__dirname, 'public/error.html'),
401: path.resolve(__dirname, 'views/error-unauthorized.html'),
// etc.
},
```

Requires #5