M-Mode only setting

[TheThirdOne]

trafficstars

A setting to use M-mode CSRs would be useful for people who deploy code to simple hardware. I would envision this working by additionally disabling environment calls and other forms of trap handling. This would result in it being impossible to stop a program from inside the program.

#97 is related as it deals with trap handling.

I made this issue because of what @phummel mentioned in #3.