Feature: %-subsitution in !/:sh

[vlofgren]

Currently helix and evil-helix does not substitute % into the current filename when running a command via :sh or !, but vim does. Helix in fact gives a parse error in this scenario, which seems like a bug more than anything else.

Allowing the vim-like behavior would go a long way toward permitting work-arounds for the current lack of plugins, since you could inform a shellscript or similar about where the editor is pointed, and have it act upon that information in some way (e.g. running unit tests in the same directory or whatever).