fix(mark2confluence): Check if mark headers are present at the beginning of the page

[gmarraff]

The function has_mark_headers was returning True when a mark header was present in the file.

However files with headers in the content, for example:

# How to push to confluence

Create a markdown file with mark headers:
     <!-- Space: FOO ->
     # Page Title

Were also processed, causing the mark binary to fail because it couldn't find headers.

This PR fixes it by checking the presence of mark headers in the first line of the file.