Clunky --output-format behaviour when running --convert

[CodeF0x]

When running --convert, e.g. with dotnet .\FFMP.dll -d 'C:\Users\Tobi\Videos\DerEinziche\' --output-format "watermark_{{name}}.mp4" --convert, there are some inconsistencies:

• --output-pattern needs to be replaced with --output-format. I get what's its trying to achieve, but now placeholders won't work anymore
• --convert writes all processed files into the same directory the source files are at. You end up with seemingly every file twice, as the only difference is the file extension

My proposal is to re-use --output-pattern because:

• it allows you to use the extensive templating syntax
• you're able to specify a different output directory from the input directory
• rather specific use cases like adding a watermark can be served as well. When doing that with --convert, you'll end up with the "the files look identical on first glance" problem, and without you're forced to supply --codec what re-encodes the file unnecessarily

[CodeF0x]

Another issue with --output-format is that --output-pattern and --output-format are not validated mutually exclusive which can lead to confusion as in #22