Add support for interpreting footers and turning them into links

[bachp]

Is your feature request related to a problem? Please describe.
We use conventinal commits and make use of footers to link to external systems.

It would be nice to have the possibility to proccess these footers and transform them into links.
These links can then be included in the changelog entry so that readers of the changelog can just click them
to get to the linked item.

Given the following example commit:

fix(db): use correct schema relation

Detailed decription...

Fixes: #123
Fixes: workitem 456
Refs: RFC1234

I would like to generate the following changelog entry:

• fix(db): use correct schema relation (#123, workitem 456, RFC1234)

Describe the solution you'd like
I think the most flexible way would be to have an additional custom tera filters available that can be applied to each footer value during template processing.

 {% for footer in footers %}
         {{ footer | into_links }}
 {% endfor %}

The behaviour of into links would be configured in the config file as a series of replace patterns.

Describe alternatives you've considered
The following are alternatives I have tought off:

• A regex_replace in tera, as proposed in How to regex replace strings? #14, could most probably solve the issue.
• Do a prost processing on the resulting changelog file and replace the URLs there.
• The possibility to define custom filters in the config would also work, but this would need som form os scripting language support.