API: `Styler.hide_columns` replaced by `.hide_values` which also operates row-wise
For showing and hiding columns/rows `Styler` currently has two methods:

 - `.hide_columns(subset)`: which hides a selection of columns and the associated data values.
 - `.hide_index()`: which hides the index keys completely and displays all the data values.

This PR adds a `.hide_values(subset, axis, show)` method, which is a superset of `.hide_columns`. It allows the same functionality to operate row-wise, and adds the kwarg `show` which allows an inverse method, exclusively showing instead of hiding.

### Follow-on:

#### Option 1

Keep `.hide_values(subset, axis, show)` as is and **deprecate** `.hide_columns`.

#### Option 2

Make `._hide_values(subset, axis, show)` a private module and include/add the public methods:

    - `hide_columns`      = `_hide_values(subset=subset, axis="columns", show=False)`.
    - `hide_rows`         = `_hide_values(subset=subset, axis="index", show=False)`.
    - `show_columns`      = `_hide_values(subset=subset, axis="columns", show=True)`.
    - `show_rows`         = `_hide_values(subset=subset, axis="index", show=True)`.

