In execute(), check if command is Runnable()
In execute(), check if command is Runnable()

A corner case exists where c.Runnable() is not checked
before c.Run() is called, thus a nil c.Run is executed
leading to "panic: runtime error: invalid memory address
or nil pointer dereference".

This patch adds an extra c.Runnable() check in execute()
to catch that corner case.

Fixes #37.

anthonyfok/cobra,sdomino/cobra,akutz/cobra,gosuri/cobra,bep/cobra,jrossi/cobra,vikstrous/cobra,nicrioux/cobra,simlelievre/cobra,abhishekgahlot/cobra,spf13/cobra,liggitt/cobra,k4rtik/cobra,eparis/cobra,jlowdermilk/cobra