feat: use cobra to provide subcommands, move sources to lib (#506)

- Use cobra in order to provide subcommands `serve` and `db`.
  - Subdir `cmd` is removed.
  - Subdir `cli` is created, which is a standard cobra structure.
- Sources related to the core are moved to subdir `lib`.
- #497 and #504 are merged.
- Deprecated flags are added. See https://github.com/filebrowser/filebrowser/pull/497#discussion_r209428120.
- [`viper.BindPFlags`](https://godoc.org/github.com/spf13/viper#BindPFlags) is used in order to reduce the verbosity in `serve.go`.

cli/cmd/root.go

@@ -0,0 +1,75 @@
+package cmd
+
+import (
+	"log"
+	"strings"
+
+	homedir "github.com/mitchellh/go-homedir"
+	"github.com/spf13/cobra"
+	v "github.com/spf13/viper"
+)
+
+var cfgFile string
+
+// rootCmd represents the base command when called without any subcommands
+var rootCmd = &cobra.Command{
+	Use:     "filebrowser",
+	Version: "(untracked)",
+	Aliases: []string{"serve"},
+	Short:   "A stylish web-based file manager",
+	Long: `Command 'serve' is the default. Filebrowser is started
+with the provided envvars, flags and/or config file. For example:
+
+filebrowser -c config.json -p 80 -s ./srv
+
+File Browser is a static binary composed of a golang backend and
+a Vue.js frontend to create, edit, copy, move, download your files
+easily, everywhere, every time.`,
+	//	Run: func(cmd *cobra.Command, args []string) {},
+}
+
+// Execute adds all child commands to the root command and sets flags appropriately.
+// This is called by main.main(). It only needs to happen once to the rootCmd.
+func Execute() {
+	checkRootAlias()
+	if err := rootCmd.Execute(); err != nil {
+		panic(err)
+	}
+}
+
+func init() {
+	cobra.OnInitialize(initConfig)
+	rootCmd.SetVersionTemplate("File Browser {{printf \"version %s\" .Version}}\n")
+
+	rootCmd.PersistentFlags().StringVarP(&cfgFile, "config", "c", "", "config file (defaults are './.filebrowser[ext]', '$HOME/.filebrowser[ext]' or '/etc/filebrowser/.filebrowser[ext]')")
+}
+
+// initConfig reads in config file and ENV variables if set.
+func initConfig() {
+	if cfgFile == "" {
+		// Find home directory.
+		home, err := homedir.Dir()
+		if err != nil {
+			panic(err)
+		}
+		v.AddConfigPath(".")
+		v.AddConfigPath(home)
+		v.AddConfigPath("/etc/filebrowser/")
+		v.SetConfigName(".filebrowser")
+	} else {
+		// Use config file from the flag.
+		v.SetConfigFile(cfgFile)
+	}
+
+	v.SetEnvPrefix("FB")
+	v.AutomaticEnv()
+	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+
+	if err := v.ReadInConfig(); err != nil {
+		if _, ok := err.(v.ConfigParseError); ok {
+			panic(err)
+		}
+	} else {
+		log.Println("Using config file:", v.ConfigFileUsed())
+	}
+}