Adding option to omit icons

Do not show icons in quotes if user configures an `omitIcons` option to true in book.json.

Example:
```
{
	"plugins"   	: ["richquotes"]
	,"pluginsConfig":
	{
		"richquotes" :
		{
			"omitIcons" : true
		}
	}
}
```

If `omitIcons` is missing or false, the icons are shown.