Support markdown checkbox.

[roachsinai]

Versions

RedNotebook version: 2-17.1

Operating system and version:Manjaro 19.0.1

Expected behavior

support Markdown checkbox!

• [ ] this

and

• [x] this

Actual behavior

[vlisivka]

x is the letter, so it will confuse spell checker and pollute tag cloud. Moreover, x can be confused with ☒.

It's better to support following notation:

• [ ] TODO
• [+] Ok
• [-] Fail

And render them as ☐, ☑, and ☒.

• ☐ TODO
• ☑ OK
• ☒ Fail

PS.

Or just add a hotkey to insert emoji, e.g. Ctrl-E + e for empty checkbox, Ctrl-E + o for OK, Ctrl-E + f for Fail, etc.

[jendrikseipp]

This issue will be automatically fixed if we add Markdown support in #560.