Prevent erroneous overwrites of target files

[OliverCi]

Prevent erroneous overwrites of Markdown files, which have already been manually edited, after they were converted using freeplane2md.

When converting, Insert a diagnostic HTML comment with a timestamp. Do not overwrite an existing target file, if its modification time is newer than the timestamp; instead exit with a message.

New command line options:
--no-timestamp: Do not insert the HTML timestamp comment.
-n --no-clobber: Prevent from overwriting an existing target file in any case, even if it does not contain a timestamp comment.
-f --force: Force overwriting the target file anyway.