Allow with_plugins() to accept a string

[geowurster]

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

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:

from click_plugins import with_plugins

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