feat(logging): make logging configurable

The log level may be set to `none`, `performance` or `debug` via the
plugins.links.logLevel property.
feat(asset-links): resolve asset links in img[src]

img[src] and a[href] may now specify links to file assets via `ref:asset:`.
These links will not be checked for existence (yet).
chore(project): bump project version to 2.8.1