Implement "Show tasks only when they contain specified tags" setting

[Perlunas]

This could be a lower hanging fruit comparing to the proper per-task filtering.

This would let us use tasks as "pseudo-database-records" if we add tags to them (partly solving request #23).

Currently, there are multiple ways to do per-file filtering (e.g. folders, filename, YAML fields, tags), but not a single way to target the tasks.

You could pass "query" into renderFileTasks function, extract the required tags and simply not render the tasks if they don't have the tags. A bit of code to redo "from and/or/not" logic but still not too complicated.

[blacksmithgu]

Full task support with tag filtering, on-complete, and so on is upcoming. I'll also fix a few bugs with the view which may cause task checkboxes to not work properly, since you should be able to check them to check the original task...

[blacksmithgu]

I have good news and bad news. The good news is this is possible; the bad news is it requires a tiny bit of JavaScript:

```dataviewjs
dv.taskList(dv.pages("#project").file.tasks.where(t => t.text.includes("#tag")));
```

[hstagner]

Is it possible to list the pages that the tasks come from as well? This works beautifully, but it is only rendering the task itself. I'd like the extra context of pages, if possible.

[hstagner]

It also only appears to re-render on file open. Is that a bug?

[hstagner]

I have good news and bad news. The good news is this is possible; the bad news is it requires a tiny bit of JavaScript:

```dataviewjs
dv.taskList(dv.pages("#project").file.tasks.where(t => t.text.includes("#tag")));

Is there a way to combine this with tasks that are not completed so that only those that are not completed show?

[blacksmithgu]

Yes.

```dataviewjs
dv.taskList(dv.pages("#project").file.tasks.where(t => t.text.includes("#tag") && !t.completed));
```

[dpurnomo]

Yes.

```dataviewjs
dv.taskList(dv.pages("#project").file.tasks.where(t => t.text.includes("#tag") && !t.completed));

I notice that this will only catch if the task is without indentation. If the task is indented (like in a nested task list) it does not work for me. Perhaps a bug?

Example:

- [ ] Test task #tag → this works
    - [ ] Test task 2 #tag → This does not work

[sheeley]

@blacksmithgu looking at this, it appears we should have an in, includes, or contains function. Did you have a preference?

[blacksmithgu]

We should add an inline in operator: 6 in [1, 2, 6], #tag in file.tags, and so on. There is already a non-inline contains(file.tags, #tag) for people who prefer that syntax, and lists will be getting a contains method later: file.tags.contains(#tag).

[blacksmithgu]

Yay, this is finally doable! Thanks to @sheeley for the implementation!

TASK WHERE contains(text, "#tag")