Support for handles with . fullstop/periods

Hello,
Gitlab supports a wider range of handle styles,
Specifically we have ones along the lines of `@person.uk`.

Having set:
```
# _config.yml
jekyll-mentions:
  base_url: https://gitlab.com
```
mentions of `@personuk` sucessfully refer to gitlab, unfortunatly `@person.uk` do not!

Any thoughts on supporting this, or where I might start in the code?
Happy to explore putting in a PR?