Make "prepend" behaviour add the template at byte zero instead of before the node

[IvanSanchez]

This is related to #3.

My use case is wanting to have no blank lines at the beginning of the file before the notice template.

In order to achieve this, I create a RegExp starting with ^, like:

var copyrightTemplate = fs.readFileSync('bsd-license.txt').toString();
var copyrightMatch = new RegExp( "^" + escapeStringRegexp(copyrightTemplate) );

// ...
	"rules":{
		"notice/notice":["error",{
			"mustMatch": copyrightMatch,
			"template": copyrightTemplate,
		}]
	}

If my files start with a few blank lines and then a block comment, then the fix will prepend the template to the first node in the AST. Unfortunately, the first node does not start at byte 0, so it leaves the blank lines at the beginning intact.

The fix uses insertTextBeforeRange([0, 0] to add the template at the very beginning of the file, instead of before the first node.

[nickdeis]

Thank you @IvanSanchez for your pull request. This fix does make more sense!