added the argument for SeqlevelsStyle and trackline in bedgraph

[ClarissaFeuersteinAkgoz]

I used write_bedgraphs, but realized two issues that were bothering me:
1- the created file was called bedgraph, but did not include the trackline.
2- my data has the seqlevelsStyle of NCBI but I needed UCSC style for uploading files for visualization in the UCSC browser.

so I have two suggested change in the function, where:

1. you can select the SeqlevelsStyle you want to have in the bedgraph file.
2. It adds the trackline within the bedgraph

Best :)
P.S.: changes in the description of the function are still missing.

[PoisonAlien]

Thank you @ClarissaFeuersteinAkgoz !