add xclip support for clipboard copy and paste functions

[kewlfft]

Description

New feature.
Basic, add xclip support for clipboard copy and paste functions

Fixes issue #5017

TODOs:

May need to indicate an optional dependency to xclip and provide information in the wiki. Let me know if I can help. Nothing else has been done so far.

[faho]

Merged, thanks!

May need to indicate an optional dependency to xclip and provide information in the wiki.

So far I've not been sure where we should document this kind of stuff. So far, we haven't documented the xsel or pbcopy/pbpaste paths either. Would a note buried somewhere in the docs really help? Would an "optional dependencies" section? Or should we add separate sections for these _commands (i.e. a fish_clipboard_paste and a fish_clipboard_copy section in the html/man page)?

Neither of those sounds particularly effective to me - I figured people would check type fish_clipboard_copy, and see that it's quite self-explanatory. Maybe my assumption is wrong though.

Where would you expect this documentation to be? I don't really want to use the github wiki for this if we can avoid it, though.

[kewlfft]

Personally I didn't look into the manual or the wiki, I directly checked fish_clipboard_copy which is the most efficient way. It is the best way to let people understand exactly what it does and contribute if they think they can do better I think.
I vote for no documentation.