Vcf functions

[BvAdrichem]

This update contains convenient functions (with documentation) for the visualization and analysis of VCF files.

The following functions have been created:

• geom_variant() [a modified ggproto of geom_point, which plots the location of mutations]
• position_variant() [a positional adjustment based on the type of mutation]
• scale_shape_variant() [default shapes for type of mutation]
• scale_color_variant() [default color for type of nucleotide]
• read_vcf() [Enables the user to read VCF file formats:: automatically recognized when using read_feats()]