Anchors do not work when the section header contains an emoji

Anchors do not work when the section header contains an emoji.

**Steps to reproduce the behavior**

A table of contents contains the link:

``` markdown
[🎯 Features](#-features)
```

The document contains the section header:

``` markdown
## 🎯 Features
```

**Expected behaviour**

When rendered using GitHub (or some other Markdown viewer) clicking on the table of contents link transports the user to the corresponding header.

**Actual behaviour**

The link does not work.

On GitHub the header link is:

``` none
https://github.com/#-features
```

Whereas this plugin produces:

``` none
http://localhost:8709/1#-features
```