msr_safe support

[fearful-symmetry]

Our underlying MSR lib supports https://github.com/LLNL/msr-safe , and we might want to add that support downstream to gorapl. Considering that this is a 1% use case, I'm still unsure about how implement this without adding extra baggage to the API as a whole.

[fearful-symmetry]

Wondering if the best way to do this is a separate set of *WithOptions() functions.