Add option to set environmental variables for cargo commands

[ChocolateLoverRaj]

I want to use Web BLE, which requires the environmental variable RUSTFLAGS=--cfg=web_sys_unstable_apis. (More info). Currently, it works if I do RUSTFLAGS=--cfg=web_sys_unstable_apis npm start, but I would prefer to have the environmental variable as part of the Webpack config so I can just run npm start.

[ChocolateLoverRaj]

I'm going to make a PR for this