Adds the option to pass a transform function to format the log record before sending it to logEntries and adds some tests

[masimplo]

Hello, I had the requirement to format the log string before sending it over to log entries. I forked your repo and added some functionality that is not specific to my case and I think that others will benefit from this. I also linted the code and added some basic tests.

ChangeLog

• Adds an optional transform function that can be passed in the options argument
• Adds a default log level that can be passed in the options argument (not sure if a default should be set anyway or throw when a level is not provided)
• Adds some basic tests
• Added package.json scripts for running jshint, tests and coverage
• Updated Readme with a more advanced example

[nemtsov]

Awesome! Thank you.