Ignore assigning the loop variable to another variable by default

[karamaru-alpha]

The default setting detects cases where you want to you aim to preserve the state prior to modification within a for loop as below.

for _, v := range slice {
   before := v

   // Some processing for the variable v.

   log.Printf("before:%+v, after:%+v", before, v)
}

We have the option "ignore-alias" to avoid to detect assignment to a different variable, but I think the presence of false positives in the default settings is not good.

So I'd like to make the default setting not to detect places assign to a different variable.
In addition, I'll create the option to detect assignment to a different variable for cases like _i := i.

Related: #8