Implemented IOManager.run_command().

Here is the method definition:

    def run_command(self, command, capitalize_first_character=True):
        process = self.make_subprocess(command)
        lines = [line.strip() for line in process.stdout.readlines()]
        if not lines:
            return
        lines.append('')
        self.display(
            lines,
            capitalize_first_character=capitalize_first_character,
            )

The method makes the right type of subprocess, runs command and then displays
output.

This cleans up quite a few places where this pattern was duplicated.