Add support for ignore files persistently

[joseluisq]

It would be great if we can support a command to ignore (or revert) files on demand and place them into .gitignore file.

Proposal: I suggest to add an ignore command.

1. Ignore a file:

~> ignore README.md
~> cat .gitignore
# README.md

1. Revert ignored file:

~> ignore --no README.md
~> cat .gitignore
# (empty)