Edits to gen_sgRNA, filter for N variants and RNA seq

[allgenesconsidered]

Added -r and -d flags for making RNA sequences for sgRNA outputs and removed sgRNA sequences for guides without PAMs. Also, filter guides for variants that fall in 'N' positions of PAMs.

Still to do, handle other IUPAC letters (etc, 'K') for guides that aren't allele-specific.