Add option to disallow pagebreaks after row

[obi134]

Add option to disallow pagebreaks after row by adding allow_pagebreak with default value = True. When disallowed, "*" is added at the end of row after "\"

with doc.create(LongTable("|l|l|")) as data_table: 
        data_table.add_hline()
        data_table.add_row([11,21], allow_pagebreak=False)
        data_table.add_hline()
        data_table.add_row([21, 22])
        data_table.add_hline()

\begin{longtable}{|l|l|}%
\hline%
11&21\\*%
\hline%
21&22\\%
\hline%
\end{longtable}%