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/doc.go

@@ -0,0 +1,77 @@
+/*
+Package filebrowser provides a web interface to access your files
+wherever you are. To use this package as a middleware for your app,
+you'll need to import both File Browser and File Browser HTTP packages.
+
+	import (
+		fm "github.com/filebrowser/filebrowser"
+		h "github.com/filebrowser/filebrowser/http"
+	)
+
+Then, you should create a new FileBrowser object with your options. In this
+case, I'm using BoltDB (via Storm package) as a Store. So, you'll also need
+to import "github.com/filebrowser/filebrowser/bolt".
+
+	db, _ := storm.Open("bolt.db")
+
+	m := &fm.FileBrowser{
+		NoAuth: false,
+		Auth: {
+			Method: "default",
+			LoginHeader: "X-Fowarded-User"
+		},
+		DefaultUser: &fm.User{
+			AllowCommands: true,
+			AllowEdit:     true,
+			AllowNew:      true,
+			AllowPublish:  true,
+			Commands:      []string{"git"},
+			Rules:         []*fm.Rule{},
+			Locale:        "en",
+			CSS:           "",
+			Scope:         ".",
+			FileSystem:    fileutils.Dir("."),
+		},
+		Store: &fm.Store{
+			Config: bolt.ConfigStore{DB: db},
+			Users:  bolt.UsersStore{DB: db},
+			Share:  bolt.ShareStore{DB: db},
+		},
+		NewFS: func(scope string) fm.FileSystem {
+			return fileutils.Dir(scope)
+		},
+	}
+
+The credentials for the first user are always 'admin' for both the user and
+the password, and they can be changed later through the settings. The first
+user is always an Admin and has all of the permissions set to 'true'.
+
+Then, you should set the Prefix URL and the Base URL, using the following
+functions:
+
+		m.SetBaseURL("/")
+		m.SetPrefixURL("/")
+
+The Prefix URL is a part of the path that is already stripped from the
+r.URL.Path variable before the request arrives to File Browser's handler.
+This is a function that will rarely be used. You can see one example on Caddy
+filemanager plugin.
+
+The Base URL is the URL path where you want File Browser to be available in. If
+you want to be available at the root path, you should call:
+
+		m.SetBaseURL("/")
+
+But if you want to access it at '/admin', you would call:
+
+		m.SetBaseURL("/admin")
+
+Now, that you already have a File Browser instance created, you just need to
+add it to your handlers using m.ServeHTTP which is compatible to http.Handler.
+We also have a m.ServeWithErrorsHTTP that returns the status code and an error.
+
+One simple implementation for this, at port 80, in the root of the domain, would be:
+
+		http.ListenAndServe(":80", h.Handler(m))
+*/
+package lib