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/bolt/config.go

@@ -0,0 +1,26 @@
+package bolt
+
+import (
+	"github.com/asdine/storm"
+	fb "github.com/filebrowser/filebrowser/lib"
+)
+
+// ConfigStore is a configuration store.
+type ConfigStore struct {
+	DB *storm.DB
+}
+
+// Get gets a configuration from the database to an interface.
+func (c ConfigStore) Get(name string, to interface{}) error {
+	err := c.DB.Get("config", name, to)
+	if err == storm.ErrNotFound {
+		return fb.ErrNotExist
+	}
+
+	return err
+}
+
+// Save saves a configuration from an interface to the database.
+func (c ConfigStore) Save(name string, from interface{}) error {
+	return c.DB.Set("config", name, from)
+}