docs: add JSDocs for top level API objects (#140)

This commit add JSDoc documentation to the CloudEvent and HTTPReceiver
objects exposed by the API when using the top level imports, specifically
`CloudEvent` and `HTTPReceiver`.

This adds a `generate-docs` npm script to generate site and API documentation
for GitHub pages in `./docs`.

Signed-off-by: Lance Ball <lball@redhat.com>

Author: lance

Diff for: .eslintrc

@@ -13,7 +13,7 @@
     "arrow-parens": ["error", "always"],
     "arrow-body-style": ["error", "as-needed"],
     "prefer-template": "error",
-    "max-len": ["warn", { "code": 80 }],
+    "max-len": ["warn", { "code": 120 }],
     "no-unused-vars": ["warn", {
       "argsIgnorePattern": "^_$|^e$|^reject$|^resolve$"
     }],

Diff for: .jsdoc.json

@@ -0,0 +1,29 @@
+{
+    "tags": {
+        "allowUnknownTags": true,
+        "dictionaries": ["jsdoc"]
+    },
+    "source": {
+        "include": ["lib", "package.json", "README.md"],
+        "includePattern": ".js$",
+        "excludePattern": "(node_modules/|docs|examples|coverage|test)"
+    },
+    "plugins": [
+        "plugins/markdown"
+    ],
+    "templates": {
+        "referenceTitle": "cloudevents-sdk",
+        "disableSort": false,
+        "collapse": true,
+        "resources": {
+          "Concepts": "concepts.html"
+        }
+    },
+    "opts": {
+        "destination": "./docs/",
+        "encoding": "utf8",
+        "private": true,
+        "recurse": true,
+        "template": "node_modules/jsdoc-fresh"
+    }
+}