Add exception for other RegExp related constants

[JDeuce]

I have some code like this:

var (
	someRegex        = regexp.MustCompile(`(?P<foo>[.+])`)
	someRegexIndex    = someRegex.SubexpIndex("foo")
)

which the linter considers OK on the regexp.MustCompile line but not on the SubexpIndex line.
Not sure it's an easy decision to add this as I know there's a slippery slope here, just wanted to share for discussion relating to #41

[gandarez]

I have kinda the same problem. Any variable of type *regexp.Regexp is ignore and a map is an issue for this linter.