Fix Styler._translate failing on numeric columns
Addresses https://github.com/pydata/pandas/issues/12125.

The previous behaviour was to use a `__getitem__` lookup which caused incorrect behaviour with numeric columns. The adjusted code uses `iloc` numerical indexing to resolve the issue.

tests!

and u can use .iloc[r, c]
 I'm working on the tests right now, just trying to figure out the best way to do "doesn't raise error" without comparing it to the output dictionary. 

Didn't know about the `.iloc[r, c]`, I'll change it to use that instead.
 I've got a branch that changes the template and translate around a bit to be cleaner.  I'll push that later today when I'm at a computer and see how it messes up yourPR here. 
 Let me know what happens and I'll rebase form your PR if this still exists. 
 It might be worth keeping the test case anyway.
 @HHammond I have got that change included on [this branch](https://github.com/TomAugspurger/pandas/tree/style-display-format). I've cherry-picked your test, thanks. I should have that branch wrapped up sometime this week.
 Sounds good. Is it time to close this PR?
 Sure, we'll leave the issue open though. Thanks.
