implement rudimentary caching for yahoo finance to avoid http 429 rate limiting

providers/staticjsonYahooFinance/cache.go

@@ -0,0 +1,15 @@
+package staticjsonYahooFinance
+
+import (
+	"time"
+)
+
+const cacheAgeSeconds = 900
+
+// intialized in providerImpl.go's Configure
+var chartCache map[string]chartCacheEntry
+
+type chartCacheEntry struct {
+	Chart       chartResponse
+	LastUpdated time.Time
+}