[ReSpec] new <rs-changelog> custom element

Hi ReSpec users,

tl;dr, we invite folks to beta test and provide feedback on a new custom element: 

```
<rs-changelog 
  from="git tag or commit"
  to="git tag or commit" 
  filter="nameOfFilerFunction">
</rs-changelog>
```

It generates a commit log list from git commits. 

Usage docs and screenshot:
https://github.com/w3c/respec/wiki/rs-changelog

Full details below

====
We are excited to introduce our first ReSpec custom element: "<rs-changelog>". Now available for *beta* testing 🔬 in . 

The `<rs-changelog>` custom element to show a list of commits between two commits or tags. This list of commits is shown from newest to oldest. Each commit message is linked to the commit.

## Attributes

 * `from`: a commit hash or git tag, from when (inclusive). 
 * `to`: optional, a commit hash or git tag until when (inclusive). If omitted, it just does to the last commit. 
 * `filter`: the name of a JS function to call, which allows you to filter out commits. 

## Filtering
The filter function takes one argument, which is a commit object. The object is composed of two properties:

 * `commit` - the commit hash. 
 * `message` - the headline of the commit message. 

### Filtering example

```JS
function respecChangelogFilter(commit) {
  // commit.hash = commit hash
  // commit.message = commit message headline
  // Return `true` if commit is to be shown, false otherwise.
  return !commit.message.startsWith('chore');
}
```

## Usage examples

```html
<p>Commits since "CR":</p>
<rs-changelog from="CR"></rs-changelog>

<p>All commits between "CR" tag and a commit "5b1a9da":</p>
<rs-changelog from="CR" to="5b1a9da"></rs-changelog>

<p>
  Show commits since "CR", but filter the commits to be shown using a
  function called `respecChangelogFilter` which is defined globally:
</p>
<rs-changelog from="CR" filter="respecChangelogFilter"></rs-changelog>
```

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