Add cp parameter to .il statement (caption placement)

Hi Roger,

In a recent project I wanted the ability to place an illustrations caption above it rather than below. This change allows that by adding a new flag to the .il statement, cp (caption placement or position). There are two possible values for this parameter (t)op and (b)ottom. If the parameter is not supplied it defaults to bottom (current behavior). 

In the future, I was thinking about adding an additional possible value (s)plit that would allow for captions to be placed above and below an illustration like

```
                   Fig 1.
+----------------------------------------+
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|                                        |
|                                        |
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+----------------------------------------+
          This is an example of a
               split caption.
```