Option to exclude all generated files

[fzerorubigd]

There is a lot of code generator in go, and also there is a convention for the generated code to identify themselves as generated code. sometimes we simply want to skip the coverage for them, so it is a good idea to add a switch to ignore them.

This is very easy to detect see https://github.com/golang/lint/blob/85993ffd0a6cd043291f3f63d45d656d97b165bd/lint.go#L125

[dave]

Excluding files based on a regex to apply to all comments might be a useful feature...