feat(ShowdownComponent): add support for template reference variable
feat(ShowdownComponent): add support for template reference variable

Set template reference variable
```html
<showdown #sd></showdown>
<div #sd="showdown" showdown></div>
```

yisraelx/ng2-md,yisraelx/ngx-showdown,yisraelx/ng2-md,yisraelx/ngx-showdown