Fix parser failing with irregular description syntax

[Blixibon]

This fixes the FGD parser failing when it encounters either a trailing + at the end of a description or an input without a description defined.

For example, the following two cases occur in the default halflife2.fgd that cause the FGD to be cut off:

 	fadescale(float) : "Fade Scale" : 1 : "If you specify a fade in the worldspawn, or if the engine is running
 	under dx7, then the engine will forcibly fade out props even if fademindist/fademaxdist isn't specified." +
+	" This scale factor gives you some control over the fade. Using 0 here turns off the forcible fades." +

	input Crouch(void) : "Crouch down"
	input Stand(void) : "Stand up from crouch"
+	input DisableCrouchWalk(void)
+	input EnableCrouchWalk(void)

I tested this and it parses the entire stock HL2 FGD with the descriptions (or lack thereof) intact, but I'm not particularly familiar with pyparsing, so let me know if this wasn't the correct way of fixing this.