Support short and long style parameters in ArgParser

[Alpar Juttner]

This is a follow-up of ticket:104.

I would propose a member function like void style(StyleEnum st), where the parameter st would choose from the following options:

• Short style, where the form of the parameters are

  -param value
  

• Long style, where the form of the parameters are

  -param=value
  

• Mixed style, where options starting with one dash use the short form while those starting with two use the long style.

With this approach, one can implement POSIX compliant parameters.