Markdown Support and Url parsing

[csoni111]

Resolves #48

• Adds Markdown support by using showdown
• Also converts any url in the message to <a href="url" target="_blank">url</a>

[csoni111]

Now the code to process markdown in the message is on the server side, instead of on the client side. Also I have added a list of allowed html tags and attributes for respective tags. This will limit markdown support and also help cope xss attack.

[csoni111]

If a user passes <del>Hello</del> into message box. This will be seen in the chat as <del>Hello</del> and not as Hello . It's because on passing it through the showdown js, the html tags will be escaped like this <del>Hello</del>. So no harm can be done!

[captn3m0]

@csoni111 Anything left on this?

[csoni111]

@captn3m0 Not from my side! I was hoping someone would merge it.