Add support for custom mutator only in libfuzzer

Hi!

Currently libfuzzer only supports using a custom mutator alongside the default mutation, however this is bad for certain types of programs which expect a highly structured input and which reject the input even if there is the smallest syntax error. I assume this code here:

```

MutationDispatcher::MutationDispatcher(Random &Rand,
const FuzzingOptions &Options)
: Rand(Rand), Options(Options) {
DefaultMutators.insert(
DefaultMutators.begin(),
{
{&MutationDispatcher::Mutate_EraseBytes, "EraseBytes"},
{&MutationDispatcher::Mutate_InsertByte, "InsertByte"},
{&MutationDispatcher::Mutate_InsertRepeatedBytes,
"InsertRepeatedBytes"},
{&MutationDispatcher::Mutate_ChangeByte, "ChangeByte"},
{&MutationDispatcher::Mutate_ChangeBit, "ChangeBit"},
{&MutationDispatcher::Mutate_ShuffleBytes, "ShuffleBytes"},
{&MutationDispatcher::Mutate_ChangeASCIIInteger, "ChangeASCIIInt"},
{&MutationDispatcher::Mutate_ChangeBinaryInteger, "ChangeBinInt"},
{&MutationDispatcher::Mutate_CopyPart, "CopyPart"},
{&MutationDispatcher::Mutate_CrossOver, "CrossOver"},
{&MutationDispatcher::Mutate_AddWordFromManualDictionary,
"ManualDict"},
{&MutationDispatcher::Mutate_AddWordFromPersistentAutoDictionary,
"PersAutoDict"},
});
if(Options.UseCmp)
DefaultMutators.push_back(
{&MutationDispatcher::Mutate_AddWordFromTORC, "CMP"});

if (EF->LLVMFuzzerCustomMutator)
Mutators.push_back({&MutationDispatcher::Mutate_Custom, "Custom"});
else
Mutators = DefaultMutators;

if (EF->LLVMFuzzerCustomCrossOver)
Mutators.push_back(
{&MutationDispatcher::Mutate_CustomCrossOver, "CustomCrossOver"});
}

```

is responsible for the selection of the mutators. A simple addition would be to add an option to only use the custom mutators.

Is this possible? I may be able to implement this myself since this seems quite simple.

Thanks in advance!