feat: dates, deadlines, weight

[alirezaalavi87]

For being able to actually plan tasks, there is a need for time management, deadlines etc.

this can easily be added by a custom tag, the same way we have tag:, [tags:],[meta-tag:], we can have something like [deadline:] which receives a valid date and can be filled with a simple html datepicker.

the same can be said about weight of a task.

we can even inspire from todo.txt

[BaldissaraMatheus]

This syntax actually looks really good, I was not familiar with todo.txt (looks like there is also todo.md). I'll definitely look into it

[BaldissaraMatheus]

Also, related to #84, #144

[benjaminapetersen]

I like the idea of a wrapping identifier around things like tag: like you posted, such as [tag: string, string, string].
I also like the idea (see the related #144, #145) of utilizing front matter --- or other mechanisms to "hide" the metadata from the UI in rendered mode (can still always see this data in Markdown mode).

Also mentioned in the other thread that a normalized data model might be valuable.

Love the brainstorming! Will be fun to see how this lands.