Better default behavior for pretty_table formatting

[seantbaker]

Right now, if a user doesn't pass in a string or dict for the col_formats argument, we use a default dictionary of {column name: formatting}.

But, what if users want to format a specific column, and don't care much about the rest? We shouldn't toss out all of the other default formats, like we do now; we should only override the format(s) supplied by the user.