Support SimpleNote checklists

[catherineomega]

SimpleNote recently released support for GitHub Markdown-style checkable lists: https://simplenote.com/2019/02/06/a-better-way-to-keep-track-of-your-tasks-check/

Is there any interest in adding support for these or for extending support for nvALT's existing @done syntax to allow them as well? (I get the impression emulating SimpleNote's emoji-clicking method might be a bit much given how nvALT is structured.)

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