Add option "indentedCodeblocks: true"

To help with post-processing with layouts, an option can be added that will result in stripping 4 or more leading spaces following a newline. Eleventy already has such an option. It helps with post-processing markdown files with a templating language.

No file, entry point, or test is named in the issue. Start by locating where plugin options and Markdown files are processed, then identify the existing test coverage for indentation behavior. Done means the new option is available and its four-or-more-leading-space behavior is verified.

Written by the indexing model from the issue text.