Feature request: drop all rows containing value
When chaining multiple calls I often want to select certain elements. Unfortunately getting only the values requires assignment and thus breaks the chaining:
`df_sub = df[df == x]`

One hack is:
`df.replace({x: np.nan}).dropna()`

I think a `drop_values()` function would be nice addition where I could say:
`df.drop_values(x).more().chaining()`
