Add functionality to get variant triplet context from a list of variant positions and the ref sequence

[kgori]

[baezortega]

Maybe the easiest would be to use a C++ library for this? But I guess it would need to be compiled during install. I haven't seen any packages for doing this fast enough outside Bioconductor, but maybe there's a package for fast string processing that we can adapt.

[baezortega]

NB. Rsamtools::scanFa can be used to retrieve regions from a FASTA file.
https://rdrr.io/bioc/Rsamtools/man/scanFa.html