add class=„pandas-empty“ to NaN-cells’ HTML
This PR is related to #5330, but is much simpler.  There is no new API, this just allows styling empty cells using CSS, e.g. using

``` css
  td.pandas-empty { color: #eee }
```

to de-emphasize, or

``` css
  td.pandas-empty { background-color: yellow }
```

to let empty cells stand out.

@cpcloud @TomAugspurger  ?
 I should mention that I _do_ see a weakness in the condition (testing the formatted contents against self.fmt.na_rep).  However, currently the design seems to make it very hard to do better; I tried implementing this before and failed, and found this simple "solution" in the above-mentioned PR #5330.
Although it is not very clean, I personally feel that currently, it is an appropriate approximation (for testing the raw value against NaN).
 @hmeine I see what you want to do here. I think we are looking for a more general soln (w/o too much refactoring / and no API breaking).Their are some ideas/suggestions in those linked threads. Just need someone to try an implementation.
 I see, but I would say those are complementary goals.  Marking empty cells (and index/column headers, cf. #6488) would be totally non-invasive and scratch obvious itches.  The more general solution would be far more capable, but the above allows _today_ to add CSS styling e.g. in IPython notebooks (which I am using daily).
 @y-p created an excellent base from which to start, in #5763 

check out this notebook (from that PR): http://nbviewer.ipython.org/github/pydata/pandas/blob/8c4f9571c08d5122f1d9e40c43dadab8b6f10bf5/pandas_HTML_styling.ipynb

in #3190 you can see all the linked issues and how many times people have requested little additions or wished pandas did some formatting thing slightly different. i totally get where you're coming from, but i think that most of the legwork was done by @y-p and we should take advantage of it rather than going for the needs of "today". if we make this change we have to support it for the foreseeable future. personally, i'd rather support an addition that is more general.
 :+1: on using the soln @cpcloud just pointed, its way more general, not that hard, and almost done. IIRC just needs some tests. Would be willing to put this in once integrated on an experimental basis. Maybe under `core.html` namespace (so can add stylers in the future)
 @cpcloud the actual PR is at #5763 
 yep my bad thx for correcting
 was gh autocomplete + enter :/
 Ah, now I see that PR #5763 is actuallly much closer to what I proposed here than the template engine proposed in PR #3190.  Then, I understand why you would rather see the generic solution.  Unfortunately, I cannot help with testing ATM.

I think this PR can then be closed.
