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

[stianhoiland]

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