Allow alternative plugin directory

[janssen-io]

Since I've started versioning assemblies and plugins are always loaded from the same directory, debugging a development build can be troublesome. The published plugin expects a specific version of the TransactionQL.Input assembly, which of course is not available in the development build directory.

Originally posted by @janssen-io in #7 (comment)

Possible fix is to set an alternative plugin directory. Initially via command line arguments, but it could also just be a setting in the About-window.