Make sorting keys configurable

[WhyNotHugo]

For example, to sort entries by line number, use:

require("trouble").setup({
  sort_keys = {
    "filename",
    "lnum",
    "col",
    "severity",
  }
})

The defaults remain the same.

Fixes: https://github.com/folke/trouble.nvim/issues/65

[WhyNotHugo]

Any feedback on this?

[WhyNotHugo]

Thanks!

[folke]

Thank you!