Index rows and columns

[dmfigol]

Currently:
– sheet.get_row() does rest api request for every call, this could be eliminated by adding a row index to Sheet
– To get a cell value by column name, a user needs to build their own index for the header. This can be part of the library and the user would be able to use provide column title to get_cell when is used on the row

I have this functionality implemented (though without tests), and I can submit PR, but I don't see much activity in this repo, so I would like to understand if it is even worth investing my time into this (and someone will review and merge PR), or I should continue using my fork for the projects.