Feature Request: Support piping buffer content to/from external tools

Nobody has claimed this yet.

The one feature to rule them all is the capability of participating in the wider environment of CLI tools by allowing to

1. run a command line in a subshell from `edit` and insert its output into the current buffer (or replace the selection, when there is a text selection), or
2. pipe/serve the contents of the current buffer (or selection) to the same and insert or replace it with the output, and
3. a macro language to simulate user input (possibly just raw VT!).

I personally believe this extension method is superior to the usual extensions and scripting language method. Want to use a scripting language? Just feed the buffer state to *any* scripting language's interpreter and write your script to output macro instructions. Now you support Python, Lua, PowerShell, Ruby, whatever.

Relevant: #17, #90

Read related issues #17 and #90 first; this issue names no files, tests, or entry points. Done requires an agreed scope for subprocess input/output, buffer or selection replacement, and macro input, plus verification of those behaviors.

Written by the indexing model from the issue text.