Added PageDown Markdown editor.

[maximz]

This commit references #80 - it is an attempt to change the Markdown editor to use PageDown.

Haven't tested this commit, though (I'm having trouble getting the DB to configure properly).

[andrewtobin]

Hey @maximz - thanks for the submission and sorry it's been such a delay in getting to it, we're going to try and get to PR's quicker.

Since we've accepted some others, yours won't merge with the latest well, so if you could merge with the latest for us and then re-send it'd be appreciated.

Also, I'm seeing some weird behaviour and the preview isn't showing when I did try your code, can you confirm that the image below is incorrect to what you sent in and I'll try again.

Also, your PR sent in some changes to the markdown engine, but didn't remove the old markdown engine, could you please include that also?

Thanks.