ENH: short-circuit config lookup when exact key given GH5147
Related #5147, #5457

before 

```
In [4]: %timeit -r 10 pd.get_option("display.max_rows")
10000 loops, best of 10: 53.3 us per loop
```

after

```
In [1]: %timeit -r 10 pd.get_option("display.max_rows")
100000 loops, best of 10: 5.5 us per loop
```

jeff's version with no error checking or rerouting deprecated keys

```
In [1]: %timeit -r 10 pd.core.config._get_option_fast("display.max_rows")
1000000 loops, best of 10: 792 ns per loop
```

Probably makes no practical difference.
