configure for cache/temp file directory

[sbwtw]

In cargo, we can change the direcoty of target and build temp files by target-dir.
My question is where I can set a dir to save RLS temporary files.

[dbr]

Same issue as #527

[Xanewok]

Fixed with #793, we can use „rust.target_dir” config option to specify desired target/ path for RLS.