Add support for Conditional Formatting

Hey,

Thanks for making this library, it has significantly lowered the bar to working with Google Sheets for me. It would be awesome to have support for Conditional Formatting on columns.

https://developers.google.com/sheets/api/samples/conditional-formatting

It looks like it could fit well in to the `range` style notation, with some kwargs for the formatting options. If I get a chance I will create a PR.

Start with the linked Google Sheets API conditional-formatting sample and inspect how gspread represents range styles. Determine how column conditional formatting and its formatting options should fit the proposed range-style notation. Done means users can create conditional formatting for columns through the library.

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