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`.


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cli/cmd/version.go

@@ -0,0 +1,26 @@
+package cmd
+
+import (
+	"text/template"
+
+	"github.com/spf13/cobra"
+)
+
+var versionCmd = &cobra.Command{
+	Use:   "version",
+	Short: "Print the version number of File Browser",
+	Long:  `All software has versions. This is File Browser's`,
+	Run: func(cmd *cobra.Command, args []string) {
+		// https://github.com/spf13/cobra/issues/724
+		t := template.New("version")
+		template.Must(t.Parse(rootCmd.VersionTemplate()))
+		err := t.Execute(rootCmd.OutOrStdout(), rootCmd)
+		if err != nil {
+			rootCmd.Println(err)
+		}
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(versionCmd)
+}