level fixes

    - fix incorrect handling of INFO
    - allow logging to be disabled via OFF or DISABLED
    - more efficient level checking
    - flag MojoX::Log::Declare compatibility gotcha
    - test case-insensitivity
    - fix doc typos

This is virtually the same as the [previous pull request](https://github.com/companieshouse/Log-Declare/pull/3), but includes a fix for the Hypnotoad issue, which was actually a [MojoX::Log::Declare compatibility](https://github.com/companieshouse/MojoX-Log-Declare/blob/d76f15344628bc830a4d4f0f75d0c84c787a9cb6/lib/MojoX/Log/Declare.pm#L16) issue (fixed by restoring `@Log::Declare::level_priority`).