Allow bcolz kwargs to pass thru

`expectedlen` is very important here. `bcolz` chooses the chunk size based on the `expectedlen` which may not be known by `blaze` or expensive to compute so the user should be able to pass this information through if they have access to it

@mrocklin what do you think about possibly inferring the `expectedlen` where we can? for backends where the row count is cheap to compute we could do this when folks are appending to or creating a new `bcolz` table.