Add the option to filter out a list of countries

[nerik]

[manicmarvin]

I think this is related to #22

Where i wrote

The ability to set a location (or better a set of possible locations) manually would be very important for many of us. If the purpose of the tool is ore than a theoretical exercise, there are many factors that determine where a meeting can feasibly be held. The political stability of the destination, the logistical support the organisers have in place, the visa-compatibility between the destination and attendees.

While the tool cannot probably account for all these factors, allowing users to manually force a set of options, will allow us to choose the least of many evils while we account for the human and other aspects of these large decisions.

PS There is something to be said for a tool that also looks at visa possibilities for this kind of planning.