Rewrite of the entrypoint in Golang (#71)

- General improvements
    - Parallel download of only needed files at start
    - Prettier console output with all streams merged (openvpn, unbound, shadowsocks etc.)
    - Simplified Docker final image
    - Faster bootup
- DNS over TLS
    - Finer grain blocking at DNS level: malicious, ads and surveillance
    - Choose your DNS over TLS providers
    - Ability to use multiple DNS over TLS providers for DNS split horizon
    - Environment variables for DNS logging
    - DNS block lists needed are downloaded and built automatically at start, in parallel
- PIA
    - A random region is selected if the REGION parameter is left empty (thanks @rorph for your PR)
    - Routing and iptables adjusted so it can work as a Kubernetes pod sidecar (thanks @rorph for your PR)

internal/constants/splash.go

@@ -0,0 +1,13 @@
+package constants
+
+const (
+	// Annoucement is a message annoucement
+	Annoucement = "Total rewrite in Go with many new features"
+	// AnnoucementExpiration is the expiration time of the annoucement in unix timestamp
+	AnnoucementExpiration = 1582761600
+)
+
+const (
+	// IssueLink is the link for users to use to create issues
+	IssueLink = "https://github.com/qdm12/private-internet-access-docker/issues/new"
+)