Add option to remove block indent in blocks with verbatim content

[mojavelinux]

When including snippets of source code from external files, the code will often be padded with a leading block indent. This leading block indent is relevant in its original context. However, once inside the documentation, this leading block indent is no longer needed.

Instead of seeing:

    def names
      @name.split ' '
    end

We want to see:

def names
  @name.split ' '
end

Introduce an attribute named indent to blocks with verbatim content (listing, literal, source, verse, etc) that allows the leading block indent to be stripped and, optionally, a new block indent to be specified. When indent is 0, the leading block indent is stripped. When indent is > 0, the leading block indent is stripped, then a block indentation equal to the value of indent is inserted.

For example, this AsciiDoc source

[indent=0]
----
    def names
      @name.split ' '
    end
----

should produce:

def names
  @name.split ' '
end

This AsciiDoc source:

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

should produce:

  def names
    @name.split ' '
  end

Note that the relative indentation between the lines of source code should not be affected.