Add -city flag

The -city flag provides a shortcut to calculate / lookup Lat/Long
values from the World Cities Database as available via

    https://simplemaps.com/data/world-cities

Author: mgumz

cmd/astral/cities.go

@@ -0,0 +1,27 @@
+//go:generate go run ../../data/citycsv2go.go ../../data/worldcities.csv.gz cities_gen.go
+
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+func latLongFromCity(city string) (lat, long float64, err error) {
+
+	city = strings.TrimSpace(city)
+	city = strings.ToLower(city)
+	for _, c := range cities {
+		ncc := strings.ToLower(c.NameCC)
+		if strings.HasPrefix(ncc, city) {
+			return c.Lat, c.Long, nil
+		}
+		// "new york" -> "newyork"
+		ncc = strings.ReplaceAll(ncc, " ", "")
+		if strings.HasPrefix(ncc, city) {
+			return c.Lat, c.Long, nil
+		}
+	}
+
+	return 0, 0, fmt.Errorf("not found")
+}