Add a line mode to the editor

[vinc]

This PR is a test to see how useful it would be to have a tiny subset of ed/vi commands in the editor.

[vinc]

Testing a substitute command:

[vinc]

Now with regex support for the substitute command:

(note that the g flag is gone, we need to also add Regex#replace_all for it)

[vinc]

The --command transform the editor into a kind of "stream editor" like sed and it would be easy to do alias sed "editor --command".