Allow multiple twitter users to be displayed in one feed.

Multiple twitter usernames can be specified in a pipe separated list.
Tweets from each of these users will then be shown in a single feed.

Example:
```
{exp:twitter:user screen_name="twitter|Support" limit="3" retweets="no"
replies="no"}
<li><a href="https://twitter.com/{screen_name}/"><strong>@{screen_name}</strong></a> - {text}</li>
{/exp:twitter:user}
```