add WithTraceAt() method to adjust Trace() log level

[sjbarag]

Hey @tj! I'm loving Apex's log module so far, and figured I'd take a crack at adding a method to allow .Trace() and .Stop() to log non-errors at a configurable level. I'm happy to rename that method as well — let me know what feels right to you :)

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The Trace() and Stop() methods for an Entry default to Info-level logging, and previously didn't offer a way to log non-errors at any other level. Add a WithTraceAt() method to allow that level to be configured.

fixes #93