Insert markdown on enter

[mxstbr]

Closes #9

[mxstbr]

@brianlovin @uberbryn mind testing this patch?

# Get repo if you haven't already
git clone git@github.com:withspectrum/draft-js-markdown-plugin.git
cd draft-js-markdown-plugin
# Install deps
yarn
# Run demo
yarn run dev
# Now open localhost:3001 in the browser and try it out!

[brianlovin]

Works like a charm! Nice 👌