var-naming: handle possible panic

[alexandear]

This PR fixes a panic when running revive with the following configuration:

[rule.var-naming]
arguments = [[], [], [{ extra-bad-package-names = ["helpers", 123] }]]

Before:

❯ ./revive -config revive.toml ./...
panic: interface conversion: interface {} is int64, not string

goroutine 1 [running]:
github.com/mgechev/revive/rule.(*VarNamingRule).Configure(0x1053e2ec0, {0x140001d37d0, 0x3, 0xa?})
        /Users/alexandear/src/github.com/mgechev/revive/rule/var_naming.go:94 +0x554
github.com/mgechev/revive/config.GetLintingRules(0x140001cdab0, {0x10540a7e0, 0x0, 0x8?})
        /Users/alexandear/src/github.com/mgechev/revive/config/config.go:158 +0x3c8
github.com/mgechev/revive/revivelib.New(0x140001cdab0, 0x0, 0x0, {0x0, 0x0, 0x140000021c0?})
        /Users/alexandear/src/github.com/mgechev/revive/revivelib/core.go:56 +0x240
github.com/mgechev/revive/cli.RunRevive({0x0, 0x0, 0x0})
        /Users/alexandear/src/github.com/mgechev/revive/cli/main.go:55 +0x8c
main.main()
        /Users/alexandear/src/github.com/mgechev/revive/main.go:7 +0x28

After:

❯ ./revive -config revive.toml ./...
initializing revive - getting lint rules: cannot configure rule: "var-naming": invalid third argument to the var-naming rule: expected element 1 of extraBadPackageNames to be a string, but got type int64

Follows #1404

[ccoVeille]

Thanks for fixing, I was surprised to see the previous PR was removing the check on string slice,by assuming a slice of any would be OK