Ability to Add Optional Tags To Notes

Tags should be saved in the frontmatter something like this:

```yaml
---
...
tags: ["tag1", "tag2", "tag3"]
---
```

The CLI should accept tags using a flag.

```bash
$ zettl note note1 -t tag1 -t tag2 -t tag3
```

While we're at it, the `zettl list` command should also be able to filter with a specific tag.

```bash
$ zettl list -t tag1
```