Flexibility x and y axis for snap tolerance.

[dustindall]

Hi @jsvine ,

I was working with a table where i needed to additional snap_tolerance flexibility on the y-axis relative to the x-axis. Therefore, I added a snap_x_tolerance and snap_y_tolerance parameters in the same fashion as text_tolerance and intersection_tolerance. I went ahead and submitted a pull request.

Thanks,
Dustin

[jsvine]

Done! Cf.: #51