Allow the user to select a portion of the map and output a png file

[SamuiNe]

In cases of users needs to save the map to their phone for offline viewing or printing, I think this might be useful feature to have. However, I think this feature might be appropriate for later stages in the project, and might be an extra feature in comparison to other issues.

If the user can add a custom note before the png file output, I think it would be better too.