Fix Charm#reset when running on iTerm

For some reason, iTerm doesn't support resetting the
screen using 0x1b63 – the builtin `reset` command even
outputs "!p" instead of clearing the screen.

By also using the [2J escape code, we can ensure that
the screen is still reset correctly on iTerm and any
other terminals that may not support 0x1b63.

[0m was required too, as otherwise the screen would be
cleared with the current background color.