apply base plugin. adds clean Task to the project.
First attempt to make jbake version configuration dynamic

Jbake switched to asciidoctorj library in Version 2.3.1
This must be handled. To be continued...
added freemaker, pegdown and asciidoctor version into Extension
switch asciidoctor dependency if jbake version > 2.3.0
configure output and input via coventionMapping and extension properties
configure clearCache via coventionMapping and extension properties
jbake configurable by extension configuration per reflection \o/
bumped jbake version to 2.3.2. Extended Readme.
Merge pull request #7 from ancho/feature/configurable-jbake-version

Configurable jbake version