Font colors in error messages are bad in some terminals

[MrPowers]

The white color font to underline cell differences isn't great in my terminal for example. It'd be totally unreadable in a terminal with a white background.

We should try to make two improvements:

• Use default terminal colors for the given users (if that's possible), so it looks good in their terminal
• Let users customize the colors

[calum-mcg]

Determining default terminal colours is difficult and impossible in some cases. We could try to guess, but for the sake of simplicity it might be easier to do the following:

• Change white to a colour that works with most common background colours by default (green?)
• Add option to change colour scheme of the table, an optional dict perhaps e.g.:

colour_scheme = {
   default:'red',
   matched:'blue',
   underlined:'green'    
}
assert_df_equality(df1, df2, underline_cells=True, colour_scheme=colour_scheme)

We would need to add a validation step for the colour scheme dict.