[ReSpec]: added support for spec level

Hi All,
Janice Shiu, one of Mozilla's Outreachy [1] Interns for 2020, has added "level" style versioning support to ReSpec.

"Leveled" specs are generally specs that build on each other in a backwards compatible way. They generally include the text from a previous level. The CSS Working Group has a long history of using levels. 

The `level` configuration options automatically appends the level to your spec’s title and shortName. The level is an integer value greater than or equal to 0.

## Examples of usage

```JS
var respecConfig = {
level: 2,
shortName: "payment-request",
}
```

Which results in:

* ` Level 2` is appended to the title, so `Payment Request Level 2`.
* The short-name becomes `payment-request-2`.

To see an example of what it renders as:

https://user-images.githubusercontent.com/870154/75014932-91dd6c80-54db-11ea-8890-08ab2f6ac7c3.png

## Wiki page
You can see the above in our Wiki also:

https://github.com/w3c/respec/wiki/level

## 💰 Please support us 💰

If you like using ReSpec, please consider sponsoring our project. The money will go towards providing opportunities for emerging developers and designers from under represented groups, and for improving the overall quality of the software (Marcos doesn't make any money from it, promise! :)).

We've raised around US$250 so far, but we'd like to raise a few thousand to maybe fly out an maintainer to the next TPAC (if it's not cancelled, thanks Coronovirus!) and to give ReSpec a new coat of paint.

If you or your organization depends on it, please consider sponsoring us:
https://opencollective.com/respec

We are happy to also work out sponsorship arrangements to add something that's currently missing.

Kind regards,
Marcos

[1] More info about Outreachy at https://outreachy.org

Received on Tuesday, 3 March 2020 08:00:27 UTC