Feature request: cvaradd command

Adds a command `cvaradd <variable> <value>` which adjusts variable by the given value. Quite handy for binds like `bind x cvaradd s_volume 0.1; bind y cvaradd s_volume -0.1`. I don't think it adds anything that can't currently be done without infinity alias;bind lines in a cfg.

If variable is undefined, no action is taken.
If variable is non-numeric, it is treated as 0
If value cannot be parsed, it is treated as 0

Thoughts?