Narrations should have a level like with logging

[haxsaw]

When setting a narration on a function, an optional level should be allowed that determines whether or not to capture the narration fragment for that function (or method) based on the currently selected narration level option. The available levels should follow the names and semantics of the levels in the logging module, an custom levels should be available as well.

All narration should have a default level of 'INFO', which is also the level that each narration thread should default to. So in that case, narrations at the INFO level or higher would have fragments captured, and narrations with lower levels are skipped.

There should be every attempt to make skipped narrations provide a speed bonus in order to tune how narration impacts a program.