Expose data-cover-customVariable

Expose the data-cover-customVariable parameter of blanket so that grunt-blanket may be used to instrument code to be reported in the browser via blanket.
Merge pull request #6 from stu-salsbury/customVariable

Expose data-cover-customVariable
cover-flags, filename-format
fix(task): data-cover-flags optional

Default for data-cover-flags is now {} if no flags are specified.