Modified export.lua filename formats.

Added mainjob, mainsubjob, and overwrite options to the export command.

* mainjob will cause the export file to be named using Character_JOB format (eg, Myname_PLD)
* mainsubjob will cause the export file to be named using Character_JOB_SUB format (eg. Myname_PLD_DNC)
* overwrite will cause it to overwrite files with the same name in the folder instead of appending the system clock time to the filename.

These changes are intended to make it more straightforward to use the exported XML with GearCollector for users who do not use Spellcast at all, as they will no longer have to be renamed to be dumped into the Spellcast folder.
Merge pull request #996 from amackenzie/patch-1

Modified export.lua filename formats.