Follow-up for #161.

It would be nice to apply colorization when we format diffs:

• color.FgWhite for lines starting with "---" and "+++"
• color.FgGreen for lines starting with "+"
• color.FgRed for lines staring with "-"
• no color for other lines

Colorization should be implemented as a template function added to defaultTemplateFuncs, say, "colordiff", used as follows in defaultFailureTemplate:

{{ .Diff | colordiff | indent }}

This is needed because we don't want to pass colorized values to user templates, if user enabled custom templates.

Ralated: #334.