Change API from .utl.addOpt to .utl.addFlag

[nugend]

In the history of the command line parser's development, addOpt was created before addOptDef and intended to support both command line flags that would take an argument as well as command line flags that toggled an option on or off.

After using the library for a long time, the addOptDef is used far more often and addOpt is actually confused by its capability of parsing a parameter. Consequently, I want to create addFlag as a limited subset of addOpt which only supports boolean flags and eventually remove addOpt as it exists altogether.