[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.=20

The `level` configuration options automatically appends the level to =
your spec=E2=80=99s title and shortName. The level is an integer value =
greater than or equal to 0.

## Examples of usage

```JS
var respecConfig =3D {
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-11=
ea-8890-08ab2f6ac7c3.png

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

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

## =F0=9F=92=B0 Please support us =F0=9F=92=B0

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=