Explicitly exclude jupyter files

[hofbi]

Alternative solution to #33.

identify also considers .ipynb files as json, which is correct. But I am very sure nobody wants to run json formatting on jupyter files, hence why not exclude it by default.

[hofbi]

Making this a draft as I just found that --exclude-types is not yet supported here.

@asottile Let me know what you think of this solution. If you are fine with my approach, I would try to contribute the required argument to the mirror maker.

[asottile]

I already rejected this in #33