Fix some proxy-related stuff

Added WithProxiesFromUrl to CheckerBuilder

Enjoy easily loading proxies from an URL using Milky. Supports CR&LF line breaks

Pause checker while saving unchecked combos

Moved results directory creation after proxy loading & validation

That way, it doesn't stupidly create a results folder if the check won't even start

Using StringBuilder.Clear instead of creating a new one wasn't really…

… an optimization

We don't have to care about the previous instance anyway since it's being garbage collected

Slight refactoring due to boredom

Fixed an exception being thrown when a capture value was null

Added a "." (dot) at the end of each Milky exception.

Fixed an issue with proxy loading...

...causing all proxies to be marked as invalid

Switched target framework from netcoreapp3.1 to netstandard2.1