Add overwrite option

Some times is nice preven to overwrite output file after re-execute buildout. For example, we can use collective.recipe.template for generate configuration, and next collective.recipe.cmd for edit it:

```ini
[generate_config]
recipe = collective.recipe.template
input = etc/configuration.ini.in
output = ${buildout:parts-directory}/etc/configuration.ini

[edit_config]
recipe = collective.recipe.cmd
on_install=true
cmds = editor ${buildout:parts-directory}/etc/configuration.ini
```
So in situation like this, we don't want to overwrite configuration after re-execute buildout. So I added overwrite option for allows to select the expected behavior:)