proper mailto handling

[jorsn]

I'd like to add that we should not forget to support the common mailto syntax. So astroid should be able to understand astroid mailto:test@example.com?subject=test%20space.

It should not rely on parameters like -m since these cannot be set in modern Firefox for mailto links for example; apart from the fact that astroid -m mailto:test@example.com causes a wrong addressee.
-- #37 (comment)

@mxmehl Is #748 a good start for mailto handling? @dnebauer One could maybe augment it with more (non-standard?) keys like attachments.

Standard: RFC6068.

[dnebauer]

@jorsn Thanks for tagging me. I'm no longer using astroid; when development/maintenance went dormant for a few years my version got very buggy and I migrated to another MUA. I remain fond of astroid, however, and am pleased to see it has recently been getting some love.

[mxmehl]

I am also not using astroid any more, rather because my new work environment is uncompatible with astroid :(

But it looks like your PR looks like a feasible solution.