add function for covplot()

[MingLi-929]

According to issue #185 , I add function to specify the color for the covplot().

devtools::install_github("MingLi-929/ChIPseeker",build_vignettes = T)

peak <- getSampleFiles()

## the order maters
covplot(peak[4:5],fill_color = c('black','red'))

# change the order of color
covplot(peak[4:5],fill_color = c('red','black'))

## it can also specify for single sample
covplot(peak[[4]],fill_color = c('red'))

# the color can be specify in hex number
covplot(peak[4:5],fill_color = c('#00FFFF','#FF4040'))