Add option to draw multiple x axes for multi-rect plot

[mnahinkhan]

Thank you for your useful package!

Recently, I needed the ability to draw multiple X axes for the multi-track rectangular Manhattan plot (just for ease of readability).

I modified the function a bit so thought I'd contribute in case others found it useful.

I added two new parameters to the function:

• mul.xaxis - this allows you to specify whether you want multiple x axes drawn (default FALSE)
• mar.between - allows you to add more margin space between the rectangular plots (could be useful for users who want more space for readability, etc) (default 0, but mul.xaxis==TRUE adds a bit of margin on its own)

I also updated the README to reflect this change and showcase the multi-axes one, but of course feel free to remove this change.

I am open to any feedback, thanks again!

[YinLiLin]

Thank you very much for the contribution. It's now merged in this project.