`option-ext` dependency

dirs-sys 0.4.1 was just released and comes w/ a new dependency option-ext. I think this is a bit unfortunate due to two reasons:

1. dependency bloat: dirs-sys is a rather low-level crate and just pulling a unused dependency just for a bit of Option convenience seems a bit excessive. Note that option-ext was basically unused by the Rust ecosystem prior to this release:
   
2. license: while dirs-sys (and all other dirs-dev project) are licensed under Apache2 + MIT (which is the de facto default for the Rust ecosystem), option-ext is MPL which is a copyleft license.

I can file a fix PR if this is OK w/ the maintainers.