preGranges: filter GRanges and add flanking regions
In guokai8/CandiHap: Build Haplotye and Generate Figures

filter GRanges and add flanking regions

preGranges(
  gr,
  gene = NULL,
  chr = NULL,
  region = NULL,
  exon = TRUE,
  cds = FALSE,
  gene_name = FALSE,
  format = NULL
)

`gr`	GRanges object
`gene`	gene name (set to NULL if specify the chromosome name and region)
`chr`	chromosome name
`region`	a vector include the start and the end location
`exon`	a logical value indicating whether use exon information or not
`cds`	a logical value indicating whether use cds information or not
`gene_name`	a logical value indicating whether use gene_name or not(for gtf)
`format`	If not missing, should be one of “gff”,“gff3” or “gtf”.