Can't find 'markmon' command on Windows

Adding `shell=True` to the `Popen()` call fixes it.

Tracing file system activity with procmon shows that plugin_host is trying to find `markmon.exe`, then `markmon --port.exe` etc.

I'm not an expert on the subtleties of `Popen`, but it seems (from the docs) best to use a subshell if one has a complex command string. In any case, one or more of plugin_host/Python/Windows is parsing the command string incorrectly.

This has the happy side-effect (at least on Windows) of **not** opening a new shell window.