update to G304 which adds binary expressions and file joining

[cschoenduve-splunk]

This update to rule G304 gives gosec the ability to evaluate two path joining functions as well as binary expressions when inside ioutil.Readfile or os.Open. Gosec can now flag cases like:
ioutil.Readfile("/tmp/" + input + "/blob") and os.Open(filepath.Join("/tmp/", file)). To do this, a new function called FindVarIdentities was added to helpers.go. The function will return all variable identities found in a binary expression.

[gcmurphy]

Mind also adding test source code here

https://github.com/securego/gosec/blob/master/testutils/source.go#L475

[cschoenduve-splunk]

Sure! I added 3 new tests.

[gcmurphy]

Thanks. 👍