Make it possible to add new ExportFormats via extensions

If you want to add an export format for financial batches, you have to patch core CiviCRM atm, because of these code fragments:

• https://github.com/civicrm/civicrm-core/blob/master/CRM/Batch/BAO/Batch.php#L596
• https://github.com/civicrm/civicrm-core/blob/master/CRM/Financial/BAO/ExportFormat.php#L77

I have a patch that removes the need to adapt these files if you want to add a new export format; I will create a PR in a moment.

If this patch is applied, you can create new export formats in an extension, by doing the following:

• Create a new class in the extension that inherits from ExportFormat, and implements generateExportQuery, makeExport, and getFileExtension. And optionally output.
• Implement civicrm_hook_buildForm to add the export format to the CRM_Financial_Form_Export form
• Create a custom template templates/CRM/Financial/Form/Search.tpl so that it shows your Export format.

It is a first try, maybe more simplifications are possible.