Feature request: Access multiindex by name
#### Code Sample, a copy-pastable example if possible

I have not found an easy way to access a column of a multi index by name. Consider the following code:

```
import pandas as pd
df = pd.DataFrame([[1,1,10,200],
                   [1,2,11,201],
                   [1,3,12,202],
                   [2,1,13,210],
                   [2,2,14,230]],
                  columns=list('ABCD')).set_index(['A','B'])
ii = df.index

```

To access the values of column B i can e.g. do:

```
df.reset_index('B').B.tolist()
```

or 

```
[v[1] for v in ii.tolist()]
```

Both of there are somewhat cumbersome What I'm proposing is to add similar shorcut access to a multiindex just like a dataframe. E.g.

```
ii.B
```

or

```
ii['B']
```

that would return the same list as in the two examples above.
#### output of `pd.show_versions()`

0.18.1
