Add <directory> to limit what files are tested

This patch adds an optional base `<directory>` setting, useful when you want to run `docbook-linktester` only on pre-processed files with concrete links (not links that include Maven properties for example), and only on DocBook XML sources (not site files for example).

At present it's possible to end up with many "false positives" that make it harder to see what's really important in the output. For an example, see <https://builds.forgerock.org/view/Docs/job/OpenAM%20Java%20EE%20Agent%20Docs/ws/target/docbkx/linktester.err/*view*/>.