chore: configure prettier and eslint

- add prettier to do formatting
- add eslint-config-prettier to
  disable rules conflicting with prettier
- remove eslint from grunt workflow
- use lint-stage to lint and format
  on precommit
- use eslint and prettier in travis directly
- remove rules that are already part of
  the "recommended" ruleset

That rational is that eslint and prettier should be run in
Travis-CI, on commit and as IDE integration (highlighting
errors directlry). They don't need to be run along with
test-cases. Getting linting errors when running the tests
because of missing semicolons is just annoying, but doesn't
help the overall code-quality.

.gitignore

@@ -1,15 +1,18 @@
-vendor
 .rvmrc
 .DS_Store
-lib/handlebars/compiler/parser.js
-/dist/
 /tmp/
-/coverage/
-node_modules
 *.sublime-project
 *.sublime-workspace
 npm-debug.log
 sauce_connect.log*
 .idea
-yarn-error.log
+/yarn-error.log
+/yarn.lock
+node_modules
 /handlebars-release.tgz
+
+# Generated files
+lib/handlebars/compiler/parser.js
+/coverage/
+/dist/
+/integration-testing/*/dist/