Allow additional attributes to be defined on source block

[ldez]

[source,ruby,subs=attributes+]
----
source 'https://rubygems.org'
gem 'asciidoctor', '{latest-release}'
----

Fix #93

[ldez]

• Support substitution in additional parameters.
• Support for 'raw' listing block.

Before:

After:

Syntax example

[source]
----
source 'https://rubygems.org'
gem 'asciidoctor', '{latest-release}'
----

[source,fkdsmlmlkf]
----
source 'https://rubygems.org'
gem 'asciidoctor', '{latest-release}'
----

[indent=2]
----
    def names
      @name.split ' '
    end
----

[source,java,subs="{markup-in-source}"]
----
System.out.println("Hello *bold* text").
----

[source,xml,subs="attributes+,+replacements,-callouts"]
----
<version>{version}</version>
<copyright>(C) ACME</copyright>

content in 1 element
</1>
----

[source,ruby,subs=attributes+]
----
source 'https://rubygems.org'
gem 'asciidoctor', '{latest-release}'
----

[source,ruby,linenums]
----
puts "Hello, World!"
----

[nicorikken]

(The paste-able syntax example is a nice touch)

[nicorikken]

Tested, code reviewed, looks good for merge.

[mojavelinux]

:+1: