Re-enable extensions not coming from Chrome Web Store on Chrome v35+ (with enhanced security)

You can't re-enable them. You need to workaround this issue using any of these alternatives (I will list them for grade of difficulty):

Install other Chromium-like browser

Since Chromium is opensource, there are several forks of the project. I'm not sure if Chromium has the restriction in place but other project may not.

Moving from Windows/OSX altogether

This restriction is put in place for Windows and OSX due security concerns of the entire OS. Linux builds are not affected. You could give any Linux distribution a try.

There are really only a couple of options open to you as the ability to run the extensions has been programatically disabled with no plans to re-enable it (or at least none made public)

You can try installing from the canary channel or the developer channel releases which may allow you to still run these extensions as mentioned in Google Chrome help forum:

What if I want to run non-web store extensions?

Advanced users can continue to use our Dev & Canary channels to run any extension. Please note that these channels are updated very regularly, and may contain features and bug fixes that are actively being developed.

Alternatively, I have heard that quite a few people install Tampermonkey which then allows the running of user scripts.

Might be worth a look.

There's another option (at least on Mac) -- there's Google's own Chrome Apps & Extensions Developer Tool (https://chrome.google.com/webstore/detail/chrome-apps-extensions-de/ohmmkhmmmpcnpikjeljgnaoabkaalbgc/details) which allows you to re-enable any extension installed.