Support for other variable names

[anupshinde]

Great work!

You might want to add a line in the README to allow multiple variable names. I had a hard time figuring out why anything other than "x", "y","z" won't work.

https://github.com/rogeralsing/go-genetic-math/blob/master/ast/mutate.go#L11

For anybody who's looking:
To support other variable names, I modified

var letterRunes = []rune("xyz")

func randomLetter() string {
	return string(letterRunes[rand.Intn(len(letterRunes))])
}

to

var variableNames = []string{} // or []string{"x", "y", "z"}

func randomLetter() string {
	return variableNames[rand.Intn(len(variableNames))]
}

// registering variable names externally - if required
func RegisterInputVarName(variableName string) {
	variableNames = append(variableNames, variableName)
}