Feature request: Difficulty Selection

[Anopob]

As mentioned in #106, a difficulty selection feature could be great.

Some ideas on what could possibly change depending on difficulty:

• Item drops
• Secret locations (like the current "Hard" secrets mode)
• Secret rewards
• Enemy types (eg. Easy = more crows, Hard = more Barneys)
• Level order (?)
• The number of unarmed / ammoless levels

I would imagine there would be at least three difficulties: Easy, Hard, and Random. Or, if difficulty levels are not concrete, perhaps a slider of some sort? Anyways, thought I would make an issue to start some discussion.

[eycore1]

Perhaps the difficulty should be numbered 1-10 for secrets however, the problem is harder secrets are fewer and thus will be encountered more often. I don't mind them being paired with the normal difficulty secrets. We should definitely eliminate the "easiest" of secrets if the player chooses the highest difficulty.