Improve Application and Request extensions

Sources/Mailgun/Extensions/Mailgun+Application.swift

@@ -1,6 +1,12 @@
 import Vapor
 
 extension Application {
+    /// Mailgun storage, use it to configure mailgun.
+    ///
+    /// ```swift
+    /// app.mailgun.configuration = .environment
+    /// app.mailgun.defaultDomain = .init("mg.example.com", .eu)
+    /// ```
     public var mailgun: MailgunStorage {
         .init(self)
     }

Sources/Mailgun/Extensions/Mailgun+Request.swift

@@ -1,11 +1,14 @@
 import Vapor
 
 extension Request {
-    public var mailgun: MailgunStorage {
-        application.mailgun
+    /// Mailgun with default domain.
+    /// Default domain should be configured in advance through `app.mailgun.defaultDomain`
+    public func mailgun() -> Mailgun {
+        application.mailgun()
     }
 
-    public func mailgun(_ domain: MailgunDomain? = nil) -> Mailgun {
+    /// Mailgun with selected domain.
+    public func mailgun(_ domain: MailgunDomain) -> Mailgun {
         application.mailgun(domain)
     }
 }