BLD/DOC: report context on warnings and add :okwarning: to ipython_directive
I plan to get this upstream soon, unless joris find bugs.

Reports context on warnings just like recently added context for raised exceptions.
adds :okwarning: option to ipython directive to suppress them.

```
reading sources... [  6%] basics                                                                        

>>>-------------------------------------------------------------------------
Warning raised in /home/user1/src/pandas/doc/source/basics.rst at line 734:
----------------------------------------------------------------------------
/home/user1/src/pandas/pandas/core/frame.py:2805: FutureWarning: TimeSeries broadcasting along DataFrame index by default is deprecated. Please use DataFrame.<op> to explicitly broadcast arithmetic operations along the index
  FutureWarning)

<<<-------------------------------------------------------------------------
```

Let's get rid of the noise.

@y-p This is really fantastic! I tested it and it seems to work perfectly. Up to a completely warning/error free doc build.

Did you already introduce this in the source code of the docs? Hopefully not (I hadn't seen it), because I have a little PR ready.
 I was hoping that by giving you tools to painlessly handle it, you will.
Hey, it worked!  ;)
