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

README.md

@@ -100,6 +100,13 @@ Go-toml provides two handy command line tools:
     go install github.com/pelletier/go-toml/cmd/tomljson
     tomljson --help
     ```
+  
+ * `jsontoml`: Reads a JSON file and outputs a TOML representation.
+ 
+    ```
+    go install github.com/pelletier/go-toml/cmd/jsontoml
+    jsontoml --help
+    ```
 
 ### Docker image