Fix SupSub up/down after deleting a block #typo #ImAnIdiot
Fix deleting second SupSub block #ImAnIdiot

Specifically, after a block has been deleted from a 2-block SupSub, fix
deleting the newly subscript-only or superscript-only SupSub.
Add test case for tall SupSub's

Was a problem before #246 fixed it, superseding #249.
Merge pull request #248 from mathquill/fix.SupSub

Fix SupSub after deleting a block #ImAnIdiot