Configurable update interval.

[asheriif]

• Added an optional parameter to let the user pick the update interval (in milliseconds)
• If not passed, defaults to 100 ms.
• This is to address the high cpu-usage issue, which I have faced myself on an old computer.
• Fixes vtop consumes processor's time  #55 CPU usage really high 15%+  #82 Ability to set update interval? #17