Ability to append data to existing files

[fjgarlin]

It'd be great to add an optional parameter in Spreadsheet::save to append the contents of the current export to an existing file. This parameter would just add the flag FILE_APPEND to the function file_put_contents.

[mburtscher]

The current architecture does not allow such a behavior. I'll try to explain:

Writers are meant to return the content of the target file as a string since it may be sent direct over the wire with the download() method or saved to a file with the save() method. So the writer does not know whether the content will be added to a file or saved to a new file while creating the export.

For CSV and TSV it would work to just append the newly created content at the end of the file, but it won't work for more complex writers such as the ODS writer which creates a ZIP file.