Switch to coverage_nightly ?

[emarcotte]

I was trying to run coverage tests something that depends on this crate and noticed that it fails with:

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> C:\Users\Eugene\.cargo\registry\src\github.com-1ecc6299db9ec823\encase-0.4.0\src\lib.rs:2:23
  |
2 | #![cfg_attr(coverage, feature(no_coverage))]
  |   

According to https://github.com/taiki-e/cargo-llvm-cov#exclude-function-from-coverage this could be solved by a command line flag or a change to checking for coverage_nightly instead of coverage. Would a PR be acceptable to switch to coverage_nightly or is it preferable to use the command line flag for llvm-cov?

[teoxoy]

I think renaming it to coverage_nightly would be best.