Support Multi-select

[st0le]

I noticed it doesn't support multiselect, would be great if this can be supported.
I'm not much of a C guy but I'll see if I can implement it myself. Creating Issue for tracking.

Amazing tool. I'm looking forward to using it in my workflow.

[JerwuQu]

It supports selecting multiple items by pressing Ctrl+Enter, which will output the chosen line without exiting.

Selecting multiple lines to output later on is currently not supported, but I have nothing against accepting a PR for it.
I think it should be behind a -m flag which changes Enter to select items, and then Ctrl+Enter to confirm selections... or something along those lines.