Add compiler option to enable bulk memory

[abelstuker]

Closes #63.

The --disable-bulk-memory / --disable bulk-memory flags of the WAT and AssemblyScript compilers are not always desirable. Therefore, these flags are now made opt-out and can be disabled by specifying this as a CompilerOption.

Example:

spec.test({
    title: 'Test bulk memory',
    program: `bulk_memory.wast`,
    dependencies: [],
    steps: [],
    compilerOptions: { disableBulkMemory: true } # added
});