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/db.go

@@ -0,0 +1,24 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+// dbCmd represents the db command
+var dbCmd = &cobra.Command{
+	Use:     "db",
+	Version: rootCmd.Version,
+	Aliases: []string{"database"},
+	Short:   "Manage a filebrowser database",
+	Long: `This is a CLI tool to ease the management of
+filebrowser database files.`,
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Println("db called. Command not implemented, yet.")
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(dbCmd)
+}