feat!: Gate log crate behind feature flag

[joshka]

• Add log feature flag (enabled by default)
• Move log specific code to log module
• Move top level re-exports of log types to log module

BREAKING CHANGE: The log crate is now an optional dependency, enabled by
default. The log::Level and log::LevelFilter types are now
re-exported from the log module rather than the crate root. If you
were using these types directly, you will need to update your imports.

-use clap_verbosity_flag::{Level, LevelFilter};
+use clap_verbosity_flag::log::{Level, LevelFilter};

[coveralls]

Pull Request Test Coverage Report for Build 11903102822

Details

• 20 of 36 (55.56%) changed or added relevant lines in 2 files are covered.
• 8 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+2.2%) to 51.087%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/log.rs	16	32	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	8	51.67%

Totals
Change from base Build 11897580108:	2.2%
Covered Lines:	47
Relevant Lines:	92

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💛 - Coveralls