Does not support multiple headings having the same name

When used in a file where two or more headings have the same name, the generated links use the same reference for each. For example, running `org-make-toc` on
```
* Heading
* Heading
```
gives
```
- [[#heading][Heading]]
- [[#heading][Heading]]
```

Instead, it should generate something like
```
- [[#heading-1[Heading]]
- [[#heading-2][Heading]]
```