Support serde for Monzo and YNAB transaction structs

Define transaction structs for both Monzo and YNAB. Each package has a `main`
function that runs some shallow but preliminary round-trip tests for the
serializers and decoders.

The fixtures.json file that each of them is referencing has been ignored in case
either contains confidential data of which I'm unaware.

monzo_ynab/ynab/serde.go

@@ -0,0 +1,52 @@
+// This package hosts the serialization and deserialization logic for all of the
+// data types with which our application interacts from the YNAB API.
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+type Transaction struct {
+	Id           string    `json:"id"`
+	Date         time.Time `json:"date"`
+	Amount       int       `json:"amount"`
+	Memo         string    `json:"memo"`
+	Cleared      string    `json:"cleared"`
+	Approved     bool      `json:"approved"`
+	FlagColor    string    `json:"flag_color"`
+	AccountId    string    `json:"account_id"`
+	AccountName  string    `json:"account_name"`
+	PayeeId      string    `json:"payeed_id"`
+	PayeeName    string    `json:"payee_name"`
+	CategoryId   string    `json:"category_id"`
+	CategoryName string    `json:"category_name"`
+	Deleted      bool      `json:"deleted"`
+	// TransferAccountId interface{} `json:"transfer_account_id"`
+	// TransferTransactionId interface{} `json:"transfer_transaction_id"`
+	// MatchedTransactionId interface{} `json:"matched_transaction_id"`
+	// ImportId interface{} `json:"import_id"`
+	// Subtransactions interface{} `json:"subtransactions"`
+}
+
+// Attempts to encode a YNAB transaction into a string.
+func serializeTx(tx *Transaction) (string, error) {
+	x, err := json.Marshal(tx)
+	return string(x), err
+}
+
+// Attempts to parse a string encoding a transaction presumably sent from a
+// YNAB server.
+func deserializeTx(x string) (*Transaction, error) {
+	target := &Transaction{}
+	err := json.Unmarshal([]byte(x), target)
+	return target, err
+}
+
+func main() {
+	target, _ := deserializeTx(tx)
+	out, _ := serializeTx(target)
+	fmt.Println(out)
+	fmt.Println(ynabOut)
+}