Show errors with stack when DEBUG=1

[k1LoW]

$ ./tbls doc pg://postgres:invalid@localhost:55432/testdb\?sslmode=disable .
pq: password authentication failed for user "postgres"
$ DEBUG=1 ./tbls doc pg://postgres:invalid@localhost:55432/testdb\?sslmode=disable .
pq: password authentication failed for user "postgres"
github.com/k1LoW/tbls/cmd.printError
        /Users/k1low/src/github.com/k1LoW/tbls/cmd/root.go:64
github.com/k1LoW/tbls/cmd.glob..func4
        /Users/k1low/src/github.com/k1LoW/tbls/cmd/doc.go:56
github.com/k1LoW/tbls/vendor/github.com/spf13/cobra.(*Command).execute
        /Users/k1low/src/github.com/k1LoW/tbls/vendor/github.com/spf13/cobra/command.go:766
github.com/k1LoW/tbls/vendor/github.com/spf13/cobra.(*Command).ExecuteC
        /Users/k1low/src/github.com/k1LoW/tbls/vendor/github.com/spf13/cobra/command.go:852
github.com/k1LoW/tbls/vendor/github.com/spf13/cobra.(*Command).Execute
        /Users/k1low/src/github.com/k1LoW/tbls/vendor/github.com/spf13/cobra/command.go:800
github.com/k1LoW/tbls/cmd.Execute
        /Users/k1low/src/github.com/k1LoW/tbls/cmd/root.go:53
main.main
        /Users/k1low/src/github.com/k1LoW/tbls/main.go:34
runtime.main
        /Users/k1low/.anyenv/envs/goenv/versions/1.10.2/src/runtime/proc.go:198
runtime.goexit
        /Users/k1low/.anyenv/envs/goenv/versions/1.10.2/src/runtime/asm_amd64.s:2361