Add support for WSL

[xarthurx]

Since TaskWarrior is a linux only software, and can only be run in Windows on WSL/cygwin, would it be possible to add support to call those functions in native Windows?

For example, all command under WSL can be called in native Windows like:

c:\> wsl xxx

This would be great for extensions like taskwiki under vim in native Windows.

[xarthurx]

add a PR here: https://github.com/robgolding/tasklib/pull/62