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

Dockerfile

@@ -8,3 +8,4 @@ RUN go install ./...
 FROM scratch
 COPY --from=builder /go/bin/tomll /usr/bin/tomll
 COPY --from=builder /go/bin/tomljson /usr/bin/tomljson
+COPY --from=builder /go/bin/jsontoml /usr/bin/jsontoml