resolves issue #43 by promoting baseDir option.
resolves issue #31 by promoting iconsdir attribute.
Merge branch 'master' of https://github.com/asciidoctor/asciidoctor-java-integration
resolves issue #31.
resolves issue #45 by adding a parameter of type OptionsBuilder.
resolves issue #42 by adding readHeader method which only reads the
header withour parsing the whole document.