Seems reasonable to me, I always tab personally.
I’m not a fan. In my opinion, this modification comes at the expense of interoperability, which is kind of the benefit you get in return for limiting yourself to pure MarkDown.
At the same time, from my user perspective its hard to judge whether this sacrifice is worth the gain, but I feel it is, in fact, a significant one. 
I think tabs are just clearly better. Just have tabs be the way you indent/outdent lines, and in settings you can let your users decide how tabs are represented (2 spaces, 3, 4, etc.). Should be up to them. Under the hood, it’s just a tab though.
Here’s a great read.
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