Use invisimail as an API of sorts

Sometimes in custom theming and such I could reuse some of the functions of invisimail, but because of the way they pass their variables this is somewhat of a hack. This cleans that up and makes it possible to call the function as

invisimail_ascii_encode($string, $js = FALSE, $link = FALSE, $text = NULL)

Where string is the email address to be encoded, $js and $link are boolean for using Javascript and making a link, and $text is an optional string to use for the link text.

With docs.