Here's a patch with most of the requested features as well as adding a bit of my own spice here and there... Enjoy!
-Zeldars

New:
- Added "Brightness" setting
- Added "Quicksave" and "Quickload" keybinds
- Added "Toggle FPS count" keybind
- Added "Toggle HUD" keybind

Improvements:
- Improved visuals when firing with the Havoc powerup: Blue fireballs, blue explosions, new muzzle flashes, and more!
- The Chaingun will now get hot and smoke when firing continously! This doesn't affect gameplay
- Updated several particles effects
- The platforms in E2M3's blue zone are now longer
- Added some particle effects in E3M1's machine room
- The mouse control is now a bit more responsive, less floaty.
- Added some invulnerability frames when loading a game: no more insta death on bad saves!
- Setting a keybind that's already used elsewhere will now set the other keybind to 'none' and a warning will be shown

Balance Changes:
- You will now receive a bit less self damage from your own explosions, except for the Shock rifle's secondary
- Flame Cannon's rate of fire increased by 12.5%
- Chaingun's explosive mode rate of fire increased by 12%

Bug fixes:
- Fixed some enemies that could get stuck in E1M4
- Fixed a possibility to get soft locked in E1M4
- Fixed a softlock spot in E2M4
- Fixed a bug in E1M4 where it was difficult to re enter the castle
- Fixed a bug in E3M1 where a Pyro could kill you from the machine room
- Fixed a bug in E3M1 where an enemy could rarely go through the floor when crushed by a door
- Fixed a bug in E3M5 where you could go out of bounds