Vcftoolz

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* Fixed imports in our fork of pyvenn

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* Fixed broken Pyvenn dependency

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* Update PyVCF to PyVCF3

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* Fix defect in narrow command wrongly printing ALT=. when GT=.
* Add the ``count`` command to count samples, positions, calls, snps, indels,
other variants, filtered calls, missing calls, and filter reasons.
* Add the ``plot`` command to plot calls along the length of the genome and show
the location of filtered calls.
* Change the text of the compare report to refer to "Calls", not "Sample snps".
* Drop support for Python 3.4, which is not supported by matplotlib.
* Add support for Python 3.7.

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* Replace None with '.' when printing call data.
* Support VCF files with multiple alternate alleles per position.

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* Support reading gzip compressed vcf files.