Option to export CSV as UTF-8-BOM instead of UTF-8

[mtaki14]

Including the byte order mark (BOM) allows MS Excel (and potentially other programs) to recognise the file as UTF-8 automatically, without the need to go through the data import wizard to set the encoding manually.

I always wondered why files (in my case containing greek characters) exported through HeidiSql were read by Excel fine simply by opening them, but with TablePlus I always need to go through the import wizard and set the encoding otherwise they appear as gibberish. Turns out Heidisql includes the BOM and TablePlus doesn't.

(issue probably linked to #413)