As owner of > 500 purchased apps on Mac App Store and iOS App Store, and careful observation of buying trends at various price points, I'd say $4.99 is the impulse ceiling.

For iOS apps I am familiar with and use regularly, I have paid as much as $49.99; Mac apps, as much as $299.

This app, to me, in its current incarnation and marketing, looks like $4.99 "launch price" and $9.99 tops once it's finished (it doesn't look finished). There are notably (sorry) better editors for less money.

    Writeroom $9.99
    iA Writer $8.99 (iOS $0.99)
    Byword $9.99 (iOS $2.99)
    OmmWriter Dāna II $4.99

Perspective:

    Pages $19.99

Given the competition and its marketing, I'd recommend undercutting the other guys by as much as 1/2, and discounting your launch pricing further.

// Disclaimer: utterly speculative opinion, YM will V

EDIT: While researching these, I discovered "Marked", and saw it offers GitHub technical markdown preview. I purchased it for $3.99, because I hadn't spent money on coffee today.

I see that you've already lowered the price to $8.99 without enough time to get some second opinions here:

>> Special Launch Week Pricing!!! << Grab it now while it lasts.

So now I have to ask, what of the 10 or so people that purchased the app just yesterday for $14.99?? Are you refunding their money?

IMHO $3.99 is the ceiling for an impulse purchase. I would consider going higher if it fulfilled certain criteria for me but by then, it's no longer an "impulse" buy.