Support Windows Subsystem for Linux (WSL)

[x11x]

WSL is a proper Linux distro so process.platform === 'linux'. However, most people won't have an X server installed so xsel won't work.
But WSL can call native windows executables and the fallback/clipboard_*.exe programs work fine.
Thus, if we use the windows lib on WSL, everything just works.
Code for checking whether we're running in WSL is based on https://stackoverflow.com/a/43618657, which has a quote from a WSL dev as the source.

[notlmn]

There's https://github.com/sindresorhus/is-wsl you can use to check if the script is run in a WSL environment.

[x11x]

Thanks. I will change to use that.
Lint is failing too, sorry will fix that.

[x11x]

Sorry, this isn't ready to merge yet. It was a simple change and worked in my environment but I realise now that its not a complete solution.
The native fallbacks work from within WSL as I have said above, but they need to be wrapped in cmd.exe /q /c.
I have tried the shell: true option of execa but that does not seem to work. It might be simplest to just manually wrap the calls to the fallback binaries inside cmd.exe /q /c in lib/windows.js when running in WSL.
I will change that and reopen.

It worked perfectly when I was testing before because ... shells and terminal environments on Windows are complicated these days. (I have node installed in both windows and WSL, and I had clipboardy installed in both; WSL was using some component from the windows install).

[x11x]

What I said above about cmd.exe is incorrect. The problem is that the fallback binaries need to be on a letter drive. I have provided more information in #53