Add support for ignore files persistently

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:__

```sh
~> ignore README.md
~> cat .gitignore
# README.md
```

__1. Revert ignored file:__

```sh
~> ignore --no README.md
~> cat .gitignore
# (empty)
```
