[COOK-3259] Add configuration options

http://tickets.opscode.com/browse/COOK-3259

This Logwatch cookbook doesn't document any of the node attributes it allows you to specify, and furthermore it currently only allows you to specify the "MailTo" attribute.

I have added several other common Logwatch attributes to the recipe, given them sensible default values, and documented them in the Readme.

Attributes added are:
* Detail
* Range
* Output
* Format