Implement plugin subsystem

[derzhavin3016]

Plugins may help us to track a specified set of events in user code, without modifying hSim sources.
Requirements:

1. Plugins subsystem should be able to define events list in extendable way
2. Each plugin should register handlers for events in order to be called on them
3. We should have a useful interface to (un)register user-defined plugins dynamically (dlfcn.h should help)
4. Desired set of events (signatures could be discussed in personal):
   1. PreInsn
   2. PostInsn
   3. MemRead
   4. MemWrite

Later, we could implement cosimulation dump as a plugin to minimize hSim code changes