Replaced horizontal line plt.plot with plt.axhline to control style and width

[eporetsky]

I have made a minor modification to the way the horizontal lines are plotted. I wanted to be able to be able to change the line style to dashed lines instead of solid lines and replacing the plt.plot command with a plt.axhline seemed like the most straightforward solution. I have added variables to control the axhline linestyle and linewidth following your naming conventions and it seems to work well with your example tutorial notebook (updated) and an inverted manhattan plot (not included).

I thought it might be a useful small addition. Thank you for creating assocplots!

[khramts]

Thank you for making these additions!