Group anchors with HTML entities

Using aglio `2.0.4` with olio `1.2.1`, the following

``` markdown
# Group Foo™

## Bar ♥

```

leads to the following HTML

``` html

Foo™
¶

Bar ♥
¶

```

The group anchor is escaped while the other one is not. I think, HTML escaping does not make sense here ([URL encoding](https://tools.ietf.org/html/rfc2396#section-2.4.1) could). I am not sure whether Unicode in anchors is a good idea but I have no better one.

This issue may be related to #160.

Start by reproducing the issue with the supplied Markdown in aglio 2.0.4 and olio 1.2.1, then compare how group and heading anchors are generated in the rendered HTML. Done means entity-containing anchors are handled consistently without the group anchor being incorrectly escaped.

Written by the indexing model from the issue text.