Feat: support for hyperlinks

[govindsartaj]

Hyperlink support added for Markdown style links - see #6

For example, the parser will convert [Postlight Careers](https://postlight.com/careers) to:

{
    type: "link",
    anchorText: "Postlight Careers",
    href: "https://postlight.com/careers"
}

which will be rendered as a simple <a> tag:

<a href="https://postlight.com/careers">Postlight Careers</a>