feat(dependency): use official simple mde from npm

[casaper]

• use yarn package manager
• make hard links from node_modules/simplemde/diest to vendor/assets

Reason: This way the real version of simple mde can be used directly and changing the version can be handled with yarn as with any other npm package

[michelegera]

Hey @casaper, thanks for this PR!

This is actually a nice idea, but I am going to put it on hold for now as I am working on porting the plugin to another JS library (EasyMDE).

Once that’s done, I will incorporate your idea in the main repo!

[michelegera]

Superseded by 3f00c2d1b2c8c3d25ac69772aa4b33ce2108b09c