Make the default open program configurable.

Right now, we hardcode the evince program to open the pdf files. This however doesn't provide enough configurability for the user. And might not work on other operating systems.

https://github.com/phiresky/ripgrep-all/blob/2ccf3eeb71e491f074fce67e3c2d1f72f380b09a/src/bin/rga-fzf-open.rs#L14

I propose a configuration parameter we create for opening the different mime types. Even using something like a platform dependent file opener like xdg-open on linux or open on macos would be better.