Add "-s" option to delete dotenv files on command failure

Rationale: sometimes we want to secure our environment variables so that the next command cannot access them.
We can chain the two operations:

```
godotenv cmd
rm .env
```

The problem here is if the command fails, we do not remove the file.

We can code some shell:

```
godotenv cmd || rm cmd && exit 1
```

There are two problems now:

1. We lose the original exit code.
2. We have to write this for each executed command.

`godotenv -s` solves everything.

README.md

@@ -143,7 +143,8 @@ Assuming you've installed the command as above and you've got `$GOPATH/bin` in y
 godotenv -f /some/path/to/.env some_command with some args
 ```
 
-If you don't specify `-f` it will fall back on the default of loading `.env` in `PWD`
+If you don't specify `-f` it will fall back on the default of loading `.env` in `PWD`.
+Optional flag `-s` removes all the loaded files if the command fails and returns the same exit code.
 
 ### Writing Env Files
 

cmd/godotenv/cmd.go

@@ -4,28 +4,32 @@ import (
 	"flag"
 	"fmt"
 	"log"
-
+	"os"
+	"os/exec"
 	"strings"
 
 	"github.com/joho/godotenv"
 )
 
 func main() {
-	var showHelp bool
+	var showHelp, deleteFileOnError bool
 	flag.BoolVar(&showHelp, "h", false, "show help")
 	var rawEnvFilenames string
 	flag.StringVar(&rawEnvFilenames, "f", "", "comma separated paths to .env files")
+	flag.BoolVar(&deleteFileOnError, "s", false, "delete .env files if the command fails, copy exit status")
 
 	flag.Parse()
 
 	usage := `
 Run a process with an env setup from a .env file
 
-godotenv [-f ENV_FILE_PATHS] COMMAND_ARGS
+godotenv [-f ENV_FILE_PATHS] [-s] COMMAND_ARGS
 
-ENV_FILE_PATHS: comma separated paths to .env files
+ENV_FILE_PATHS: comma separated paths to .env files (.env is the default)
 COMMAND_ARGS: command and args you want to run
 
+  -s                         delete .env files if the command fails, copy exit status
+
 example
   godotenv -f /path/to/something/.env,/another/path/.env fortune
 `
@@ -49,6 +53,20 @@ example
 
 	err := godotenv.Exec(envFilenames, cmd, cmdArgs)
 	if err != nil {
+		if deleteFileOnError {
+			if len(envFilenames) == 0 {
+				envFilenames = []string{".env"}
+			}
+			for _, filename := range envFilenames {
+				fileErr := os.Remove(filename)
+				if fileErr != nil {
+					log.Println(fileErr)
+				}
+			}
+			if state, ok := err.(*exec.ExitError); ok {
+				os.Exit(state.ExitCode())
+			}
+		}
 		log.Fatal(err)
 	}
 }