Add sufficient spacing after header

[MawiraIke]

Fixes #182

Add

This code automatically calculates the height of the header table and starts writing the body (excl. header) after (+ calculated-height top-margin). One can however specify a custom height after which the body is written hence (+ custom-height top-margin). This can be used to add padding below the table. However, when the given custom height is lesser than the height of the table, the table and the content will overlap as occurs with the current code. Fair to note that the custom height provided is not used to change the height of the table, but just assumed to be the height of the table by the code, if it exists.

Example of adding padding

(pdf
  [{:header {:table
             [:pdf-table
              {:header [[[:pdf-cell {:colspan 2}
                          [:paragraph {:align :center :style :bold} "Customer Orders"]]]
                        [[:phrase {:style :bold} "Name"]
                         [:phrase {:style :bold} "Order Amount"]]]}
              [50 50]
              ["Joe" "$20.00"]
              ["Bob" "$7.50"]
              ["Mary" "$18.90"]]
             :height 130}}    ;; Add key height, this key can be omitted for automatic
   [:list {:roman true}
    [:chunk {:style :bold} "a bold item"]
    "another item"
    "yet another item"]
   [:phrase "some text"]
   [:phrase "some more text"]
   [:paragraph "yet more text"]]
  "ike-doc.pdf")