Adds custom non HTML elements to the schema
function addCustomElements(custom_elements) {
			var customElementRegExp = /^(~)?(.+)$/;

			if (custom_elements) {
				each(split(custom_elements), function(rule) {
					var matches = customElementRegExp.exec(rule),
						inline = matches[1] === '~',
						cloneName = inline ? 'span' : 'div',
						name = matches[2];

					children[name] = children[cloneName];
					customElementsMap[name] = cloneName;

					// If it's not marked as inline then add it to valid block elements
					if (!inline) {
						blockElementsMap[name.toUpperCase()] = {};
						blockElementsMap[name] = {};
					}

					// Add elements clone if needed
					if (!elements[name]) {
						elements[name] = elements[cloneName];
					}

					// Add custom elements at span/div positions
					each(children, function(element, child) {
						if (element[cloneName])
							element[name] = element[cloneName];
					});
				});
			}
		}