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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lib/staticgen/staticgen.go

@@ -0,0 +1,19 @@
+package staticgen
+
+import (
+	"errors"
+	"os/exec"
+)
+
+// runCommand executes an external command
+func runCommand(command string, args []string, path string) error {
+	cmd := exec.Command(command, args...)
+	cmd.Dir = path
+	out, err := cmd.CombinedOutput()
+
+	if err != nil {
+		return errors.New(string(out))
+	}
+
+	return nil
+}