Output Path should accept subdirectory path in string

[AMDphreak]

Would be nice to be able to specify the output path inside the output element of custom targets, not just the output filename.
https://mesonbuild.com/Custom-build-targets.html\

Filenames and file paths are generally the same idea in most programs. Remember, a file path is just a long name, intended to make all filenames unique in the filesystem.