Ids specific options:
* Remove unused ID names for elements: remove all unreferenced ID attributes.
* Shorten IDs: reduce the length of all ID attributes, assigning the shortest to the most-referenced elements. For instance, #linearGradient5621, referenced 100 times, can become #a.
* Preserve manually created ID names not ending with digits: usually, optimised SVG output removes these, but if they're needed for referencing (e.g. #middledot), you may use this option.
* Preserve these ID names, comma-separated: you can use this in conjunction with the other preserve options if you wish to preserve some more specific ID names.
* Preserve ID names starting with: usually, optimised SVG output removes all unused ID names, but if all of your preserved ID names start with the same prefix (e.g. #flag-mx, #flag-pt), you may use this option.