API extension points (#473)

* API endpoint extensions working.

extensions example.

* Added server.NewEnv and some docs for the API extensions example.

extensions example.
example main.go.

* Uncommented special handler stuff.

* Added section in docs for extending API linking to example main.go.

* Commented out special_handler test

* Changed to NewFromEnv

api/server/init.go

@@ -0,0 +1,52 @@
+package server
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+	"strings"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/gin-gonic/gin"
+	"github.com/spf13/viper"
+)
+
+func init() {
+	viper.AutomaticEnv() // picks up env vars automatically
+	cwd, err := os.Getwd()
+	if err != nil {
+		logrus.WithError(err).Fatalln("")
+	}
+	// Replace forward slashes in case this is windows, URL parser errors
+	cwd = strings.Replace(cwd, "\\", "/", -1)
+	viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+	viper.SetDefault(EnvLogLevel, "info")
+	viper.SetDefault(EnvMQURL, fmt.Sprintf("bolt://%s/data/worker_mq.db", cwd))
+	viper.SetDefault(EnvDBURL, fmt.Sprintf("bolt://%s/data/bolt.db?bucket=funcs", cwd))
+	viper.SetDefault(EnvPort, 8080)
+	viper.SetDefault(EnvAPIURL, fmt.Sprintf("http://127.0.0.1:%d", viper.GetInt(EnvPort)))
+	viper.AutomaticEnv() // picks up env vars automatically
+	logLevel, err := logrus.ParseLevel(viper.GetString(EnvLogLevel))
+	if err != nil {
+		logrus.WithError(err).Fatalln("Invalid log level.")
+	}
+	logrus.SetLevel(logLevel)
+
+	gin.SetMode(gin.ReleaseMode)
+	if logLevel == logrus.DebugLevel {
+		gin.SetMode(gin.DebugMode)
+	}
+}
+
+func contextWithSignal(ctx context.Context, signals ...os.Signal) context.Context {
+	ctx, halt := context.WithCancel(context.Background())
+	c := make(chan os.Signal, 1)
+	signal.Notify(c, signals...)
+	go func() {
+		<-c
+		logrus.Info("Halting...")
+		halt()
+	}()
+	return ctx
+}