ENH: add parameter to to_html for HTML border
Closes https://github.com/pydata/pandas/issues/11563

Previously this was hardcoded to `<table border="1" ...>`. Now we allow an option to `to_html`.

The only controversial bit here may be the option. I really don't like having options control output, but given that this is essentially a repr I think it's OK. It's also the only way to expose it to the user since `_repr_html_` can't take any arguments. I also selfishly need this [for my own library](https://github.com/tomaugspurger/stitch), where the user wouldn't be able to pass an option 😄 

Interestingly, the `border="0"` is seemingly redundant in the notebook:

![screen shot 2016-08-21 at 7 51 02 pm](https://cloud.githubusercontent.com/assets/1312546/17840966/c4942c98-67d8-11e6-8359-90f46dbb8cb7.png)

Looking at the CSS, I see jupyter applying a border-width of 0.909091px in their style, though I can't remember if I've adjusted the default CSS on this computer; will check later.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14061?src=pr) is 85.27% (diff: 100%)

> Merging [#14061](https://codecov.io/gh/pydata/pandas/pull/14061?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14061   diff @@
==========================================
  Files           139        139          
  Lines         50511      50517     +6   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43071      43078     +7   
+ Misses         7440       7439     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [10bf721...49e57e7](https://codecov.io/gh/pydata/pandas/compare/10bf7213240eaa0960f50f86c7790d64d8973060...49e57e7fc01757867a02715617931960451d8ead?src=pr)
 @TomAugspurger On second thought, should the option be in `pandas.options.html` or in `pandas.options.io.html` (for consistency with excel and hdf)? 
Consistency is nice, but in this case more flat options namespace is also nice (it's not that there are many entries in pd.options)
