Implement export formats as FileFormatProvider

[markjszy]

From Dominik Reichl:

Implement each of your export formats as a FileFormatProvider and add it to m_host.FileFormatPool? Your formats then appear in the export dialog of KeePass. Users could still export the selected entries only (via 'Entry' → 'Data Exchange' → 'Export Entry'), but additionally they could export a group or the whole database, your formats would be usable in triggers, etc. If you'd do this, maybe it would make sense to rename the plugin to 'SimpleTextExport' or something like that, because it wouldn't be limited to selections anymore.