Make Ghost Editor compatible with showdown latest

[tivie]

• turned a bunch of options on in showdown to set a "ghost" flavor markdown, similar to GFM
• custom extensions live in their own repo, with proper tests against showdown
• updated dependencies in package.json
• should fix a number of issues and bugs in markdown parsing

[tivie]

Should you choose to accept this PR, I can change ownership of the custom extensions repos to you.

[disordinary]

Thanks for that @tivie,

This will be a welcome improvement in our markdown rendering!

[tivie]

No problem. You should decide what showdown options should be turned on and off so that I can create, in showdown, a "Ghost Flavor".

This way, code would be simplified and any changes in showdown via options could be integrated, automatically, in Ghost.

[tivie]

Added one more commit to bump package.json versions.

Both showdown and extensions were tested in Ghost environment against a bigger number of tests. As a result, some minor issues and edge cases were fixed.