Reverse order of "jsonptr" function arguments

... for better integration with template pipelines.

README.md

@@ -10,7 +10,11 @@ Apply [Go templates](https://golang.org/pkg/text/template/#hdr-Text_and_spaces)
 
     goproc <Go-template-file> <JSON-or-YAML-file>
 
-## Go template functions extensions
+## Template syntax
+
+See the [Go templates](https://golang.org/pkg/text/template/#hdr-Text_and_spaces) documentation.
+
+## Functions extensions
 
 The following functions are added in addition to the standard functions.
 
@@ -19,9 +23,10 @@ The following functions are added in addition to the standard functions.
 To ease the extraction of data, `jsonptr` allows to express data location using
 JSON Pointer ([RFC 6901](https://tools.ietf.org/html/rfc6901)).
 
-Usage:
+Usages:
 
-    {{ jsonptr . "pointer" }}
+    {{ jsonptr "pointer" . }}
+    {{ . | jsonptr "pointer" }}
 
 Examples:
 

main.go

@@ -56,7 +56,9 @@ func _main() error {
 
 	tmpl := template.New("")
 	tmpl.Funcs(template.FuncMap{
-		"jsonptr": jsonptr.Get,
+		"jsonptr": func(ptr string, doc interface{}) (interface{}, error) {
+			return jsonptr.Get(doc, ptr)
+		},
 	})
 
 	var err error

testdata/02.gotmpl

@@ -1,2 +1,2 @@
-Hello, {{ jsonptr . "" }}!
+Hello, {{ jsonptr "" . }}!
 {{/* vim: syntax=gotexttmpl: */ -}}

testdata/03.gotmpl

@@ -1,2 +1,2 @@
-Hello, {{ jsonptr . "/name" }}!
+Hello, {{ . | jsonptr "/name" }}!
 {{/* vim: syntax=gotexttmpl: */ -}}