Support appending to an existing Markdown file

[zcutlip]

Optionally append to an existing Markdown file rather than creating a new one

Additional changes are around gracefully handling missing title and table of contents, such as in the event we're building on an existing file

[didix21]

Hi @zcutlip, Good job and thanks for your contribution. I can see tests are not passing. Would you mind reviewing them please? Thanks