Add markdown support

[ldidry]

With the help of marked

[ocdtrekkie]

This code works pretty great. I integrated it into 0.5 of the Sandstorm.io package of Scrumblr. I just had to grab some changes for marked.min.js to make links open in a new tab so that they'd work with Sandstorm's sandboxing: ocdtrekkie@6acde85 (This contains a hacked-together version of marked.js with a PR the marked.js author refused to merge from another user.)

It may be noteworthy that this functionality may be better outside Sandstorm as well. I doubt people who click links on their Kanban want to open them in the same tab.

[Framartin]

This PR should include the security fix of this issue when it will be ready (this is confidential for now).

[Framartin]

@ldidry Can you include the fix to this XSS in this MR.

@ocdtrekkie I have created a MR in your repo to fix the Sandstorm version: ocdtrekkie#11

[ldidry]

@Framartin Done.