Implement possibility to disable mutants via comments

[k3rn31]

It would be useful to disable specific mutations via comments.

Proposal

The comment should have the format:

//nomutant

or

//nomutant:mutation1,mutation2,...,mutationN

For example:

To exclude all mutation from specific expression:

a := b + c //nomutant

To exclude specific mutation from specific expression:

a := b + c //nomutant:arithmetic-base,invert-bitwise

To exclude mutations from block:

//nomutant
func myFunc() {
  a := b + c
}

To exclude mutations from file:

//nomutant
package apackage