fix LCOV reporter when toHTML is false

Hi,

Once we run LCOV reporter with the option *toHTML* as false, it set only the last file coverage result.

I've fixed that creating the global attribute `_$blanket_LCOV` as soon as blanket.customReporter is called, then I increment the same. I thought to implement inside the conditional `options.toHTML !== 'undefined'` before, but it can inherit old results in some cases.

An alternative to let each file result separated(the same way as html) is turning `_$blanket_LCOV` into array.

So, what do you think?

Cheers!