Some refactoring for vmtop

[pmmccorm]

• Wire in refresh option
• Add -n/--number: run n times and quit
• Pull out argument parsing into free functions from VmTop class
• Pull out signal handler + global cancel variable to free functions
• Add loop() and run() functions with exception handlers around them to relieve inentation stress on VmTop main loop
• Add main function to handle parsing + VmTop creation

[jdesfossez]

Awesome, thank you !