Support Windows when not using backtick execution

Fixes https://github.com/jekyll/jekyll/issues/2556
Add docs for Posix::Spawn#default_command_prefixes
* is not necessary there.
default_command_prefixes ~> system_command_prefixes
Nope, I totally need that star.
Need r and w in `.
Merge pull request #57 from parkr/patch-1

Support Windows when not using backtick execution