Add done mode

[nhz2]

If a stream is stopped when writing because the stop_on_end option was set, read should not suddenly start working.

To fix this I added a :done mode which is not readable. The existing :stop mode is still readable, but can only be reached through :read mode.

This doesn't fix the issues with eof described in #170

[vtjnash]

I am not familiar with the workings of the package. But a cursory glance looks good.