Allow with_plugins() to accept a string

There really isn't a need for the user to be doing:

``` python
from pkg_resources import iter_entry_points

from click_plugins import with_plugins

@with_plugins(iter_entry_points(entry.point))
```

When they could just do:

``` python
from click_plugins import with_plugins

@with_plugins('entry.point1', 'ep.2')
```
