Added feature for custom presets

Hello! I added a capability to let the user add and save a custom preset to the equalizer. The user clicks the "Add Custom" button and a prompt appears, asking the user what they wish to name this new preset. Once the user clicks OK, the state of the EQ sliders (eq[1].gain through to eq[10].gain) and the name is pushed to the PRESETS array and the name of the preset is added to the bottom of the drop-down menu. I haven't yet implemented a way for the preset to be permanently saved in the user's browser -- I believe it remains in memory for that session. I'm currently looking into improving the feature.

I initially tried to implement this feature reading a text file, but shelved the idea, and reverted "const.js" back to its prior state.