Move to yaml-based configuration

This commit updates the configuration to be yaml-based and updates the
configuration to read in a yaml file.

internal/config/validators/url.go

@@ -0,0 +1,46 @@
+package validators
+
+import (
+	"net/url"
+
+	"github.com/pkg/errors"
+)
+
+// URL represents a url.URL that is marshaled as a string representing the url.
+type URL url.URL
+
+func (u *URL) MarshalText() (text []byte, err error) {
+	return []byte(u.String()), nil
+}
+
+func (u *URL) String() string {
+	if u == nil {
+		return ""
+	}
+	return (*url.URL)(u).String()
+}
+
+func (u *URL) UnmarshalText(text []byte) error {
+	if len(text) == 0 {
+		return nil
+	}
+	pu, err := url.Parse(string(text))
+	if err != nil {
+		return err
+	}
+	*u = URL(*pu)
+	return nil
+}
+
+func ValidateHTTPSURL(url **URL, dflt string) error {
+	if (*url).String() == "" {
+		*url = new(URL)
+		if err := (*url).UnmarshalText([]byte(dflt)); err != nil {
+			return err
+		}
+	}
+	if (*url).Scheme != "https" {
+		return errors.Errorf("%q is not a valid https URL (%q)", *url, (*url).Scheme)
+	}
+	return nil
+}