Accept pathlib.Path for all functions taking a filepath

[mbhall88]

Hey team,

First of all, LOVE this package. Has made working with VCFs so much less painful for me.

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Feature request

Any function that takes a filepath should be able to acept a pathlib.Path object in addition to the current str type.

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Keep up the awesome work!

[alimanfoo]

Thanks @mbhall88, this would be a nice enhancement. May be hard to get to in the short term but if you have any bandwidth at all then a PR in this direction would be very welcome.