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

.circleci/config.yml

@@ -66,6 +66,10 @@ jobs:
                 test_name: "tomljson"
                 module: "github.com/pelletier/go-toml/cmd/tomljson"
                 allow_fail: <<parameters.allow_fail>>
+            - run_test:
+                test_name: "jsontoml"
+                module: "github.com/pelletier/go-toml/cmd/jsontoml"
+                allow_fail: <<parameters.allow_fail>>
             - run_test:
                 test_name: "tomll"
                 module: "github.com/pelletier/go-toml/cmd/tomll"