Change loglevel wrapper to use a single anonymous function

This allows for more custom integration, enabling a user
to define `this` by wrapping the loglevel code in a function
and invoking it with apply(customContext). Previously,
`this` would be null if invoked with apply.

Fixes #50
Merge pull request #50 from p-js/adjustment-for-scoping

Adjustment for scoping, so loglevel can be used with preprocessing and apply()