Fix autocomplete behavior for empty args

This also adds a modified version of go-shellwords as a dependency
and removes the dependency on the original go-shellwords.

cmd/micro/shellwords/util_windows.go

@@ -0,0 +1,20 @@
+package shellwords
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+func shellRun(line string) (string, error) {
+	shell := os.Getenv("COMSPEC")
+	b, err := exec.Command(shell, "/c", line).Output()
+	if err != nil {
+		if eerr, ok := err.(*exec.ExitError); ok {
+			b = eerr.Stderr
+		}
+		return "", errors.New(err.Error() + ":" + string(b))
+	}
+	return strings.TrimSpace(string(b)), nil
+}