Allow setting included sources to run full refresh on change

[abelsromero]

Thank you for opening a pull request and contributing to asciidoctor-maven-plugin!

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Documentation
• Refactor
• Build improvement
• Other (please describe)

What is the goal of this pull request?
Fix conversion not happending when included sources where modified in autp-refresh and http mojo.
As discussed in #403 at the end we added a new option available for those mojos to allow users to specify other files to "listen to changes on". On a change or creation, a full rebuild will be done.

Are there any alternative ways to implement this?
Current approach allows users to specify a set of extra sources to monitor.
On change a full rebuild (without copying resources) will happen. A full rebuild is necessary since we don't know which sources are affected.

Are there any implications of this pull request? Anything a user must know?

1. Creating an extension (include processor or preprocessor) that creates a tree of dependencies to know exactly which files to modify.
   Discarded because custom filters may interfere with other extensions used by users.
2. Manually parsing all sources as texts files and build such index manually.
   Discarded because configurations that maniputale paths (with basePath for example) and have files with same names may be impossible to properly resolve or we may have to also resort to "full builds".

Is it related to an existing issue?

• Yes
• No

Finally, please add a corresponding entry to CHANGELOG.adoc

[abelsromero]

I had to increase the timeout for refresh tests for ubuntu-java8. I assume they run slower on than environment for some reason.