fix(log): Use `stdpath('log')`

[ZeroKnight]

Neovim has had a standard path for log files since v0.7.0-322-g78a1e6bc0 (note this is after v0.7.2; the git describe output is a touch misleading), so this should be the preferred location. Logs aren't cache data, so they shouldn't be stored with cache data.

Given that plenary is used in a lot of plugins, this would be a tiny but free improvement in correctness for many plugins out there.

Related: #93, #487.

Fun fact: #93 was merged just 6 months before stdpath('log') landed!