Make new `dtype_coercion`='stringify' option

[deanm0000]

This is just an alternative approach to dealing with the issue outlined here

The idea being that any type can be casted to a string this would allow a file to be opened without raising on the first try without too much hackiness.

I'm not really sure which is "better" or more desirable.

[PrettyWood]

With v0.12 you can now do load a sheet with dtypes = "string" directly to set it for all columns