Patch: fix markdown indentation

This pull request (PR) first merges changes from johnfmorton@d26f72b to fix the `{::nomarkdown}{:/}` error in the output. It also fixes an issue where adding spaces for indentation to the generated code caused markdown to convert the output to an HTML code block. I added an markdown escape sequence ("\ ") and used inline string iteration over the sequence to escape the resulting output:

```Ruby
# Line: 102
# used to escape markdown parsing rendering below
markdown_escape = "\ "

# Line: 154
source_keys.reverse.each { |source|
          media = " data-media=\"#{instance[source]['media']}\"" unless source == 'source_default'
          source_tags += "#{markdown_escape * 4}<span data-src=\"#{instance[source][:generated_src]}\"#{media}></span>\n"
        }

        # Note: we can't indent html output because markdown parsers will turn 4 spaces into code blocks
        # Note: Added backslash+space escapes to bypass markdown parsing of indented code below -WD
        picture_tag = "<span #{html_attr_string}>\n"\
                      "#{source_tags}"\
                      "#{markdown_escape * 4}<noscript>\n"\
                      "#{markdown_escape * 6}<img src=\"#{instance['source_default'][:generated_src]}\" alt=\"#{html_attr['data-alt']}\">\n"\
                      "#{markdown_escape * 4}</noscript>\n"\
                      "#{markdown_escape * 2}</span>\n"

# Line: 171
source_keys.each { |source|
          if source == 'source_default'
            source_tags +=  "#{markdown_escape * 4}<img src=\"#{instance[source][:generated_src]}\" alt=\"#{html_attr['alt']}\">\n"
          else
            source_tags += "#{markdown_escape * 4}<source src=\"#{instance[source][:generated_src]}\" media=\"#{instance[source]['media']}\">\n"
          end
        }

        # Note: we can't indent html output because markdown parsers will turn 4 spaces into code blocks
        picture_tag = "<picture #{html_attr_string}>\n"\
                      "#{source_tags}"\
                      "#{markdown_escape * 4}<p>#{html_attr['alt']}</p>\n"\
                      "#{markdown_escape * 2}</picture>"
```

All changes in this branch were tested on redcarpet, kramdown, and maruku using both markup patterns.