jsontoml tool (#296)

`jsontoml` is very similar to `tomljson`
It uses json.Unmarshal to convert read json to map and then
converts the map to tree using `toml.TreeFromMap`.
Then this tree is converted to toml using `tree.toTomlString()`
The numbers when taken as input from json get converted to float64
because of how `json.Unmarshal()` converts all json numbers to float.

Fixes #280

cmd/jsontoml/main.go

@@ -0,0 +1,81 @@
+// Jsontoml reads JSON and converts to TOML.
+//
+// Usage:
+//   cat file.toml | jsontoml > file.json
+//   jsontoml file1.toml > file.json
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+
+	"github.com/pelletier/go-toml"
+)
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprintln(os.Stderr, "jsontoml can be used in two ways:")
+		fmt.Fprintln(os.Stderr, "Writing to STDIN and reading from STDOUT:")
+		fmt.Fprintln(os.Stderr, "")
+		fmt.Fprintln(os.Stderr, "")
+		fmt.Fprintln(os.Stderr, "Reading from a file name:")
+		fmt.Fprintln(os.Stderr, " tomljson file.toml")
+	}
+	flag.Parse()
+	os.Exit(processMain(flag.Args(), os.Stdin, os.Stdout, os.Stderr))
+}
+
+func processMain(files []string, defaultInput io.Reader, output io.Writer, errorOutput io.Writer) int {
+	// read from stdin and print to stdout
+	inputReader := defaultInput
+
+	if len(files) > 0 {
+		file, err := os.Open(files[0])
+		if err != nil {
+			printError(err, errorOutput)
+		}
+		inputReader = file
+		defer file.Close()
+	}
+	s, err := reader(inputReader)
+	if err != nil {
+		printError(err, errorOutput)
+		return -1
+	}
+	io.WriteString(output, s)
+	return 0
+}
+
+func printError(err error, output io.Writer) {
+	io.WriteString(output, err.Error()+"\n")
+}
+
+func reader(r io.Reader) (string, error) {
+	jsonMap := make(map[string]interface{})
+	jsonBytes, err := ioutil.ReadAll(r)
+	if err != nil {
+		return "", err
+	}
+	error := json.Unmarshal(jsonBytes, &jsonMap)
+	if error != nil {
+		return "", error
+	}
+
+	tree, err := toml.TreeFromMap(jsonMap)
+	if err != nil {
+		return "", err
+	}
+	return mapToTOML(tree)
+}
+
+func mapToTOML(t *toml.Tree) (string, error) {
+	tomlBytes, err := t.ToTomlString()
+	if err != nil {
+		return "", err
+	}
+	return string(tomlBytes[:]), nil
+}