Add travis task for markdownlint (#460)

* Add travis task for markdownlint

* Use nodejs 10

* Use `markdownlint-cli` package to run tests

* Apply to any md file

* Add config to markdownl linter to allow sibling rule

* Fix a lot of markdownlin warnings

* Use markdownlint only for CHANGELOG

* Add newline to markdown config file

.markdownlint.json

@@ -0,0 +1,3 @@
+{
+    "MD024": { "siblings_only": true }
+}

.travis.yml

@@ -0,0 +1,5 @@
+language: node_js
+node_js: "10"
+
+install: npm install -g markdownlint-cli
+script: markdownlint CHANGELOG.md