External command support

[QiBaobin]

Will there be any external command support? I think that would provide a way to do what internal command missing on early stage, like grep, git etc.

[itzmjauz]

I've always liked the vim way of ;
:!{command}

[archseer]

I think as a start, all the kakoune commands are applicable and should be easy to do:

|: pipe each selection through the given external filter program and replace the selection with it’s output.
<a-|>: pipe each selection through the given external filter program and ignore its output
!: insert command output before selection
<a-!>: append command output after selection

I'm not too opinionated on whether this should be a keymap or a :pipe command.