Replace {gradle-relative-srcdir} in gradle asciidoctor task configuration

Hi,

In #292 (v2.2.0) the plugin offered the nifty {gradle-relative-srcdir} attributes, however it can only be used in the documents themselves. I'm thinking that an improvement could be made to post process the attributes configured in the asciidoctor task in order to replace attributes such as {gradle-relative-srcdir} upon processing a (nested) document.

Otherwise it imposes to define in all documents attributes such as highlightjsdir and stylesdir, I'd rather write :

attributes = [
        'snippets'          : snippetsDir,
        'source-highlighter': 'highlight.js',
        'highlightjs-theme' : 'gruvbox-dark',
        'highlightjsdir'    : '{gradle-relative-srcdir}/highlight',
        'stylesheet'        : 'asciidark.css',
        'stylesdir'         : '{gradle-relative-srcdir}',
        'linkcss'           : 'true',
        'encoding'          : 'utf-8'
]