Check for custom cargo registry locations

[jsmnbom]

On my machine I've configured cargo to follow proper XDG paths by setting "CARGO_HOME DEFAULT="${XDG_DATA_HOME}/cargo" in my "~/.pam_environment". It could be really nice if better-panic would read $CARGO_HOME when checking if a frame is dependency code (i.e. https://github.com/mitsuhiko/better-panic/blob/master/src/lib.rs#L129). I can try to make a PR if this is something you also feel is a good idea?

EDIT: Should probably also show why i want this - as far as i can tell the "is dependency check" determines whether to trim filenames. I would like it to trim tokio file names (or maybe exclude the frame entirely as it's not really useful info):