nearest_peak: nearest_peak
In xons/chipMap: Make heatmap of High throughput sequencing data

Find nearest peak of given genes's transcription start site. If tss overlaps multiple peaks, random one will be selected.

1	nearest_peak(peak.gr, tx, tssup = 1000, tssdown = 1000)

`peak.gr`	A peak GRanges. This can be get from Bed file of peak using bed2gr function in this package.
`tx`	A data frame of transcript/gene annotation.
`tssup`	Window size around TSS that will be defined as overlap_tss, and 0 distance will be given. Default is 1000, means must overlap within 1 kb upstream of TSS.
`tssdown`	Window size around TSS that will be defined as overlap_tss, and 0 distance will be given. Default is 1000, means must overlap within 1 kb downstream of TSS.