Add support for alternative chromosome styles

[mykmal]

This update replaces the rmv_chrPrefix parameter in format_sumstats() with a chr_style parameter, as discussed in PR https://github.com/neurogenomics/MungeSumstats/pull/157.

The new chr_style parameter supports all four of the chromosome naming styles available in GenomeInfoDb (NCBI, UCSC, dbSNP, and Ensembl), with Ensembl being the default. Thus, the default setting matches the previous default behavior of MSS while more robustly handling improperly-formatted chromosome names. I have also updated all relevant documentation, vignettes, and tests.

The updated functions worked as expected on my GWAS, but it would be great if you can also double-check the code.

Note that the chr_check() function now serves to standardize chromosome names to the default Ensembl/NCBI style and remove SNPs on nonstandard or user-specified chromosomes. Chromosome name mapping to chr_style, on the other hand, is performed directly in format_sumstats() right before the final summary statistics data.table is written to disk. This order of operations is necessary because several of the checks that come after chr_check() (e.g. liftover) expect chromosome names to be formatted according to the default Ensembl/NCBI style, and I did not want to modify them.

[Al-Murphy]

This all looks good to me! Thanks again for all the work on this!