Support in-place upgrades of new minio binary and releases. (#4961)

This PR allows 'minio update' to not only shows update banner
but also allows for in-place upgrades.

Updates are done safely by validating the downloaded
sha256 of the binary.

Fixes #4781

vendor/github.com/segmentio/go-prompt/Readme.md

@@ -0,0 +1,33 @@
+
+# go-prompt
+
+ Terminal prompts for Go.
+
+ View the [docs](http://godoc.org/pkg/github.com/segmentio/go-prompt).
+
+## Example
+
+```go
+package main
+
+import "github.com/segmentio/go-prompt"
+
+var langs = []string{
+  "c",
+  "c++",
+  "lua",
+  "go",
+  "js",
+  "ruby",
+  "python",
+}
+
+func main() {
+  i := prompt.Choose("What's your favorite language?", langs)
+  println("picked: " + langs[i])
+}
+```
+
+## License
+
+ MIT