Support hiding sidebar links

[mertkahyaoglu]

Fixes #353

This PR adds a feature that allows users to hide sidebar links. You can specify the links that you don't want in the sidebar by adding hidden_links to _config.yml.

Example;

hidden_links:
  - posts
  - pages
  - ..

[ashmaroli]

Nice.. lots of changes.
Couple of suggestions:

• Since the single commit related to this PR was written after series of merge commits, use can have the merge commits removed from here by rebasing to origin/master.

# set ENV to your time of authoring commit (e.g.) 
$ GIT_COMMITTER_DATE="May 03 2017 22:24:30" git rebase origin/master

• can we have the config settings under a jekyll-admin namespace?

# _config.yml

jekyll-admin:
  hidden_links:
    - posts
    - pages
    - ..

[ashmaroli]

I see a couple of errors in the browser console when I start up server-frontend Our test suites don't cover these lines because of the shallow wrappers.