Make command-line arguments -i "" launch the portable instance

[JonathanFeenstra]

Closes #1633

The check for -i with no arguments doesn't distinguish between -i "" or just -i:

modorganizer/src/commandline.cpp

Lines 224 to 225 in 2043d99

	// handle -i with no arguments
	if (m_vm.count("instance") && m_vm["instance"].as<std::string>() == "") {

So if we want -i "" to also launch the portable instance, we need to find another way to distinguish between those cases. I don't think we really need that though.

I changed it to use boost::optional so we can distinguish between those cases now. @Al12rs didn't like using -i "Portable", since there could be a global instance named "Portable".

[Al12rs]

Were you able to test if this distinguishes between no param and empty string?

Could we use std::optional rather than the boost version or was there a reason to use boost?

[JonathanFeenstra]

Were you able to test if this distinguishes between no param and empty string?

Yes, I confirmed it's working correctly for both -i and -i "" as command-line arguments.

Could we use std::optional rather than the boost version or was there a reason to use boost?

I tried std::optional first, but unfortunately that isn't supported by boost::program_options::variables_map because it's not a streamable type.