Feature Request: Ignore certain error messages

I am using grunt-html to validate a set of HTML files which are actually HTML fragments, so I need to ignore certain messages.  I added an option called 'ignore', which accepts an array of messages to ignore.

Example usage:
```javascript
grunt.initConfig({
	htmllint: {
		options: {
			ignore: [
				'Element "head" is missing a required instance of child element "title".',
				'Start tag seen without seeing a doctype first. Expected e.g. "<!DOCTYPE html>".'
			]
		},
		all: ['content/**/*.html']
	}
});
```