Implement `getLineWidth()`

[imjosh]

I noticed that the Autotable plugin sets lineWidth but doesn't restore it to the previous value. However, there doesn't seem to be any way to check what the current value of lineWidth actually is.

[richardtorres314]

@HackbrettXXX is this good to close? Thank you!

[HackbrettXXX]

Yes, thanks.