Feature Request: pd.DataFrame drop method to include function support
I think it would be useful if the `pd.DataFrame`'s `drop` method could support a `lambda` or `function` that one can use as a filter such as below: 
Also, on a side note I noticed that masking requires an extra step in the newest pandas in that it requires the mask to be converted to an array and then to a boolean. Not sure if that warrants an issue in itself but just wanted to mention that.  It is also shown below:

![image](https://user-images.githubusercontent.com/9061708/28790400-bd7c1bf0-75dd-11e7-8af8-1a2231442081.png)
