Currently, the kubectl plugins system - when running under Windows - will determine that a plugin is executable
by looking at its extension, comparing it to '.exe'.

I think we should allow plugins w/ a few more known extensions (.bat, .com, .cmd; for example) -
This would give more of a similar experience to plugins under *nix, where shell scripts are often used.

Moreover, there were two windows-specific behaviors that were redundant,
which I've removed:

1. In plugin.go - There's no need to differentiate between windows and
under OS-es when looking at the PATH variable; It can be refered as 'PATH'
in Windows as well

2. In cmd.go - There's no need to append a suffix when looking up
plugins on windows; One can safely use 'exec.LookPath' without a suffix
on Windows - and get automatic resolving for known executable types