resolves issue #53 by changing icons parameters from boolean to string.
Now users can set which admonition icons to use.
ORIGINAL_ADMONITION_ICONS_WITH_IMG constant has been created to use the
original approach used in AsciiDoc or in Asciidoctor 0.1.2.
resolves issue #51 by adding linkattrs attribute.
resolves issue #52 by adding experimental attribute.