ENH: add option to suppress scientific notation (for small values?)
I find myself running into a situation where I don't want to see small numbers as scientific notation fairly frequently, things like:

```
In [3]: pd.set_option('display.precision', 2)

In [4]: pd.DataFrame(np.random.randn(5, 5)).corr()
Out[4]: 
      0     1         2         3     4
0  1.00 -0.57  2.15e-02 -3.48e-02 -0.64
1 -0.57  1.00  2.59e-01 -5.56e-01  0.51
2  0.02  0.26  1.00e+00  2.91e-03 -0.06
3 -0.03 -0.56  2.91e-03  1.00e+00  0.36
4 -0.64  0.51 -6.21e-02  3.63e-01  1.00
```

or

```
In [16]: pd.Series(np.random.poisson(size=1000)).value_counts(normalize=True)
Out[16]: 
0    3.80e-01
1    3.63e-01
2    1.75e-01
3    5.70e-02
4    1.80e-02
5    5.00e-03
7    1.00e-03
6    1.00e-03
dtype: float64
```

Scientific notation isn't helpful when you are trying to make quick comparisons across elements, and have a well-defined notion of a -1 to 1 or 0 to 1 range.

I propose adding some sort of display flag to suppress scientific notation on small numbers, and just report zeros in these cases instead. Alternatively we could also suppress it on large numbers, but I am not sure how helpful that is. I usually only find myself going up against it on small numbers, in exactly the use cases (correlations, proportions) above.
