Per project breakpoints

[cshjsc]

Hi, I don't know if something like this is already possible. But every time that I close a buffer, the breakpoints disappear, it would be really cool to have breakpoints per project like Harpoon saves marks for files.

[mfussenegger]

If you enable hidden the breakpoints won't disappear if you close a buffer.

There are also some snippets in #198 to persist breakpoints to disk

[cshjsc]

Thanks