Add the ability to automatically set rel and target on external links

Added three configuration values under the 'links' section:
- internalhosts: array of hostnames to ignore when deciding to add rel or target
- externalrel: value of the rel attribute that should be added to external links
- externaltarget: value of the target attribute that should be added to external links
For Markdown, inline links are processed based on the configuration values
For Twig, an 'a' function was added that takes href, text, and an array of attribute values and returns an HTML fragment with the 'a' tag. The configuration values are used to determine if rel or target should be added to external links. Additionally, it is possible to override the configuration by passing rel or target as an attribute, and passing an empty string for either will prevent the attribute from being added at all.

stakx-io / stakx