gatsby-remark-copy-linked-files with absolute path

[tc-imba]

The gatsby-remark-copy-linked-files now transform the file into a relative path.
However, I think it's better if files can be opened in a new tab (e.g., use target=_blank).

The gatsby-remark-external-links plugin can achieve this if the link is an absolute path, so maybe a new option such as "absolute" can be added to let gatsby-remark-copy-linked-files transform the file into an absolute path.