message: chore(danger): update dangerfile to validate commit message
diff: var fs = require('fs');
  var path = require('path');
  var _ = require('lodash');
  +var validateMessage = require('validate-commit-msg');
  //simple regex matcher to detect usage of helper function and its type signature
  var hotMatch = /\bhot\(/gi;
  @@ -54,3 +55,14 @@ if (testFilesMissingTypes.length > 0) {
  fail('missing type definition import in tests (' + testFilesMissingTypes + ') (' + ++errorCount + ')');
  markdown('> (' + errorCount + ') : It seems updated test cases uses test scheduler interface `hot`, `cold` but miss to import type signature for those.');
  }
  +
  +//validate commit message in PR if it conforms conventional change log, notify if it doesn't.
  +var messageConventionValid = danger.git.commits.reduce(function (acc, value) {
  + var valid = validateMessage(value.message);
  + return valid && acc;
  +}, true);
  +
  +if (!messageConventionValid) {
  + fail('commit message does not follows conventional change log (' + ++errorCount + ')');
  + markdown('> (' + errorCount + ') : RxJS uses conventional change log to generate changelog automatically. It seems some of commit messages are not following those, please check [contributing guideline](https://github.com/ReactiveX/rxjs/blob/master/CONTRIBUTING.md#commit-message-format) and update commit messages.');
  +}
  \ No newline at end of file
  