Column type specification should be more like readr or vroom

[jennybc]

The "shortcodes only" situation is going to become too limiting, very quickly.

[johnrharley]

Reading through some closed issues like #64 and #18 I'd like to throw in some support here for having an option to have the default guess for col_type go to character rather than list.

When combining dynamic and messy spreadsheets I often run into >20 columns which are imported as lists, but it can vary from sheet to sheet such that I have to sort out the atomic nature of each column individually before I can call something like bind_rows. It would be a lot easier if we could pass an option like list_as_char = TRUE so that read_sheet() defaults to an atomic type.

Thanks for continuing to develop this great package!