Add Slope property and Plot function to Equation struct

[ccampbell]

I am aware this library may not be maintained, but I was using it and wanted some additional functionality so I figured I would at least submit a pull request.

I implemented two changes:

• Added a Slope property to the Equation struct which lets you get the slope of an equation as a float using equation.Slope

• Added a Plot function to the Equation struct which lets you plot any point on the new linear approximation.

  This is so that you can pass in a bunch of points and then generate equations, and from the equations get back a new set of points that fit to the linear equations. You pass in the x coordinate is a float and get back the y coordinate.

Arguably it might be better to get back an array of all of the new points on the linear graph instead of the individual equations, but these changes made what I wanted to do easier.

Thanks for the library!

[brandenesmith]

Yeah man, I made this to write equations that were used in a constraint database system a while back. I don't maintain it or use if for that anymore, so if you are using it and want to modify it to meet your needs go right ahead. LGTM