Support forward slashes on Windows

Is your feature request related to a problem? Please describe.

I use some hyper config that I'd like to share across Windows and Unix hosts. Since Windows generally supports forward slashes, it would be nice if Hyper would do the same when on Windows, allowing a config to be shared across platforms. For example:

    shell: '/Users/jaraco/.local/bin/xonsh.exe',

That config doesn't work on Windows and will cause a crash, even if that file exists. Only after translating the forward slashes to backslashes does the application run.

Describe the solution you'd like

Better would be to honor forward slashes on all platforms, making the configuration files more portable. That could be done by translating the forward slashes to backslashes before passing them to Windows APIs... or more likely those paths can simply be used with forward slashes and Hyper merely needs to avoid crashing when forward slashes are supplied on Windows.

No file or test is named. Start by reproducing the Windows crash with the shown shell path and tracing Hyper's configuration handling; done means the same portable config runs on Windows without requiring slash translation, with coverage for this path case.