`snoop(julia_cmd)`

[vtjnash]

We allow passing flags to julia_cmd, but it would be nice to be able to pass the entire command instead. This would be helpful for SnoopCompile.jl, where we want to re-write some of the flags from julia_cmd (specifically, --sysimg and --target) and to change the cwd (might need to wait for JuliaLang/julia#24353 though)