Enable the export in unique folder instead of relative ones

[rlespinasse]

Based on this file tree

folder1/file1.drawio
folder2/file2.drawio

The run of drawio-exporter --output export --format png will produce

folder1/file1.drawio
folder1/export/file1.png
folder2/file2.drawio
folder2/export/file2.png

We want to be able to export in a unique folder and produce this kind of output (and also to manage collision if a drawio file exists in different folder)

folder1/file1.drawio
folder2/file2.drawio
export/folder1/file1.png
export/folder2/file1.png

To do so, we will add an option --output-mode (relative|absolute) to produce the output below using drawio-exporter --output export --output-mode absolute --format png. relative will be the default value and produce the current output.