Style display format
Closes https://github.com/pydata/pandas/issues/11692
Closes https://github.com/pydata/pandas/issues/12134
Closes https://github.com/pydata/pandas/issues/12125

This adds a `.format` method to Styler for formatting the display value
(the actual text) of each scalar value.

In the processes of cleaning up the template, I close #12134 (spurious 0)
and #12125 (KeyError from using `iloc` improperly)

Still a WIP for now, need to add handling for `subset` for some cases. Wanted to get this up since it's come up a couple times.
 Ok, ready for review whenever someone gets a chance. These changes should be entirely backwards compatible.
 Thanks Jeff.

I fixed everything except the `select_dtypes` enhancement. Do that as a followup since presumably it would be for all the style methods, not just `format`?
 did u update the docs with an example for this?
 Yeah, I've uploaded a [gist](http://nbviewer.jupyter.org/gist/TomAugspurger/071685a20c0a779e1ba4#Finer-Control:-Display-Values). I'm not sure why the diff was so badly mangled.

I looked into [nbsphinx](http://nbsphinx.readthedocs.org/en/0.2.3/) a bit and it looks promising. We'll need to tweak a few things to get it working with our site. I plan on doing that sometime.
 All green here.
 I need to figure out why the doc build [failed here](https://travis-ci.org/pydata/pandas/jobs/107922605). Haven't been able to reproduce it locally yet.
 lgtm. merge when ready.
 I'll get all my PRs finished up tonight. If I can't figure out why this is causing the sphinx build to fail I'll drop the changes to the notebook and do that in a followup PR.
 still bombing sphinx?
 Surprisingly, yes. Even though I removed my changes to the docs. I've got one more thing to try out this morning. 

> On Feb 11, 2016, at 21:14, Jeff Reback notifications@github.com wrote:
> 
> still bombing sphinx?
> 
> —
> Reply to this email directly or view it on GitHub.
 @TomAugspurger Looking at the doc build log, it gives an error with "ImportError: No module named abc" (for the `from collections.abc import MutableMapping` that you added). That can possibly explain also all the other problems with sphinx. 
I get that same error message if I just try the import, but strange that the other tests do not fail. (a python 2 vs 3 issue?)
 Thanks Joris! Hopefully that was it. It is strange that the tests didn't fail on py27. We do skip on the `27_slow_nnet_LOCALE` job, since we had a strange interaction between Jinja and matplotlib. Not sure why the other ones didn't fail.
 Yeah, very strange .. I checked and they are certainly not skipped on some of the python 2.7 builds
 @jreback passed without breaking sphinx, if you want to merge.
 gr8!

thanks!

and of course can just join the travis queue :)
 thanks @TomAugspurger 

I realized after I pressed Y that we don't have all of the issues listed in the whatsnew. Can you add them when you have a chance.
 Ahh yeah, sorry. I've made a todo item.
