ReSpec: linter

Hi ReSpec Users,

As part of the W3C's transition to TLS docs on /TR/, we've added a
simple linter to ReSpec to check that the URLs in your `respecConfig`
are using TLS.

The linter is disabled by default right now. You can enable it by
adding the following to your `respecConfig`:

```
lint: true,
```

You can access the warnings through the ReSpec pill on the top-right,
as per usual. If no warnings show up there, you are all good :)

The linter also warns if you don't have a Privacy and/or Security
Considerations section in your spec, with a helpful link to a
questionnaire that can help you write such sections.

Once the W3C transitions to TLS on /TR/ (in ~2 weeks), we will enable
the linter by default to help you easily fix such warnings.

Kind regards,
Marcos