ARROW-1652 [JS - WIP] Add optional batch hint to Vector.get

[TheNeuralBit]

Added an optional batch to Vector.get, as well as a *batches iterator. The most significant changes are in ListVector and IndexVector, because I removed the returnWithBatchIndex argument and made some tweaks to the way offset vectors are handled (Rather than subtracting batch when accessing data, use length - 1 to reduce `index while iterating).

[wesm]

Can you add some tests? Are there coverage tools available for Typescript?

[trxcllnt]

@wesm yep we have coverage ready to go, but commented out in the CI script until we set up something like coveralls.
@TheNeuralBit sorry I haven't responded to your comments in ARROW-1652 yet, I'll do that right now.

[wesm]

@TheNeuralBit could you add some unit tests and update the PR title with the JIRA number?

[TheNeuralBit]

@wesm I think this may be irrelevant now based on our new plans for the Vector class
@trxcllnt what do you think? Maybe ARROW-1652 should be changed to "add batch vector accessor" and include your planned changes

[trxcllnt]

@TheNeuralBit yeah that works for me, that way it's still tracked