Fix markdown-enter-key doesn't delete empty checkbox list

[kuranari]

Description

I have updated markdown-enter-key to delete empty checkboxes. I think this action is better, similar to how GitHub's issue editor handles it.

Issue

Expected Behavior:

When pressing enter on an empty checkbox list, the list should be deleted.

Actual Behavior:

A new list with a checkbox is added to the next line.

Steps to Reproduce

When I pressed enter on an empty checkbox item (with the cursor located after the checkbox),

- [x] 

a new list item was added, resulting in the following:

- [x] 
- [ ]

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] Improvement (non-breaking change which improves an existing feature)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] I have read the CONTRIBUTING.md document.
• [x] I have updated the documentation in the README.md file if necessary.
• [x] I have added an entry to CHANGES.md.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed (using make test).

[syohex]

Thanks