pc: add option to translate absolute analog values to fader input (#375)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Add a new option that allows absolute position analog input, such as
SDVX controller knobs and IIDX turntables, to be bound as input for
Polaris Chord faders.

In practice this means turning a knob counter-clockwise results in fader
being held left (relative to velocity), and vice versa.

## Testing
Tested using FauceTwo and a mouse.

src/spice2x/launcher/options.h

@@ -205,6 +205,7 @@ namespace launcher {
             MFGNoIO,
             PCArgs,
             PCNoIO,
+            PCKnobMode,
             spice2x_LightsOverallBrightness,
             spice2x_WindowBorder,
             spice2x_WindowSize,