Fixes issue #11 (nested ems)

index.js

@@ -107,7 +107,12 @@ const rules = {
 			return htmlTag('a', output(node.content, state), { href: markdown.sanitizeUrl(node.target) }, state);
 		}
 	}),
-	em: markdown.defaultRules.em,
+	em: Object.assign({ }, markdown.defaultRules.em, {
+		parse: function(capture, parse, state) {
+			const parsed = markdown.defaultRules.em.parse(capture, parse, Object.assign({ }, state, { inEmphasis: true }));
+			return state.inEmphasis ? parsed.content : parsed;
+		},
+	}),
 	strong: markdown.defaultRules.strong,
 	u: markdown.defaultRules.u,
 	strike: Object.assign({ }, markdown.defaultRules.del, {
@@ -298,6 +303,7 @@ function toHTML(source, options) {
 	const state = {
 		inline: true,
 		inQuote: false,
+		inEmphasis: false,
 		escapeHTML: options.escapeHTML,
 		cssModuleNames: options.cssModuleNames || null
 	};

test/oddities.test.js

@@ -56,3 +56,10 @@ test('Spoiler edge-cases', () => {
 	expect(markdown.toHTML('||||||'))
 		.toBe('<span class="d-spoiler">|</span>|');
 });
+
+test('Nested <em>', () => {
+	expect(markdown.toHTML('_hello world *foo bar* hello world_'))
+		.toBe('<em>hello world foo bar hello world</em>');
+	expect(markdown.toHTML('_hello world *foo __blah__ bar* hello world_'))
+		.toBe('<em>hello world foo <u>blah</u> bar hello world</em>');
+});