Custom link for contribution categories

Hi there,

I'd like to propose a feature when generating links for "ideas" contribution.
It would be handy to allow changing the link, without replacing the whole template.

Currently, the links for "ideas" are generated like this:

```
#ideas-{username}
```

I'd like to customize it to link to GitHub Issues. As this can vary from project to project, maybe `all-contributors-cli` could support link templates, per contribution category?

On `"ideas"` contributions we could have a couple of options, such as:

1. Issues created by that user:

```
/{projectOwner}/{projectName}/issues/created_by/{username}
```

2. Filtering issues with a specific label (`"ideas"` / `"proposal"` / etc)

```
/{projectOwner}/{projectName}/issues?q=author%3A{username}+label%3Aideas
```

What's your thoughts on this? Cheers!