Improved regex for markdown renderer

[GerwinvBeek]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

After the recent changes it was not possible anymore to use images in node-red markdown. The renderer would put a space between : and // when using markdown urls like ![](https://url.png).

With this change the image will render like it is supposed to.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage increased (+0.05%) to 67.394% when pulling 836f7d2 on GerwinvBeek:markdown-regex into d4d6f71 on node-red:master.

[knolleary]

Looks good - well spotted.