[MRG] Make optional arguments keyword args

[shuttle1987]

Refactor argument handling for corpus initialization, Closes #186

[shuttle1987]

Note that this is a breaking change.

The change is to this:

    def __init__(self, feat_type: str, label_type: str, tgt_dir: Path,
                 labels: Optional[Set[str]] = None,
                 *,
                 max_samples: int=1000,
                 speakers: Optional[Sequence[str]] = None) -> None:

But given we are already making a breaking change I'd rather it were this:

    def __init__(self, feat_type: str, label_type: str, tgt_dir: Path,
                 *,
                 labels: Optional[Set[str]] = None,
                 max_samples: int=1000,
                 speakers: Optional[Sequence[str]] = None) -> None:

[shuttle1987]

This is the sort of thing we should merge sooner rather than later since it will give us much more flexibility with how the API is used going forward.

[oadams]

Okay, do you think I should merge it in now? Not sure because of the WIP tag in the title.

[shuttle1987]

I made those changes, test are not updated just yet. Note that this has the capacity to break things so a version bump is probably in order if this gets merged.

[shuttle1987]

I think this is good to go now.