Naggum.Assembler: output file naming

- [Assembler.fs:53](https://github.com/codingteam/naggum/blob/96eae4f9eadcf9b0ce257cc28098bbd60fdb8bb1/Naggum.Assembler/Assembler.fs#L53)
- [Program.fs:22](https://github.com/codingteam/naggum/blob/e838e43b01aa51773c654b1665465a2eced27506/Naggum.Assembler/Program.fs#L22)

There should be a way to determine output file names for Naggum.Assembler (and also compile either `.exe` or `.dll`).
