`--pdb` option not implemented

[SebCorbin]

Django 3.1 introduced a --pdb option effectively using another result class, PdbDebugResult.

django-rainbowtest's RainbowTextTestResult is not compatible with that option for now.

The code has a commented line

    def addError(self, test, err):
        unittest.TestResult.addError(self, test, err)
        if self.showAll:
            self.stream.writeln(colors.red("ERROR"))
        elif self.dots:
            self.stream.write(colors.red('E'))
            self.stream.flush()
        # self.debug(err)

It would be nice to have a RainbowDebugResult class to implement the option