Add fontsize option to pdfpcnotes package

Backported from
https://github.com/cebe/pdfpc-latex-notes/commit/c90dd140277e40545a78a8b89df7ae0284a3b224

beamertheme-source/pdfpcnotes.dtx

@@ -45,6 +45,7 @@
 \DeclareStringOption{starttime}
 \DeclareStringOption{endtime}
 \DeclareStringOption{lastminutes}
+\DeclareStringOption{fontsize}
 
 \ProcessKeyvalOptions*
 %    \end{macrocode}
@@ -71,6 +72,7 @@
  \PDFPC@option{start_time}{\PDFPC@starttime}
  \PDFPC@option{end_time}{\PDFPC@endtime}
  \PDFPC@option{last_minutes}{\PDFPC@lastminutes}
+ \PDFPC@option{font_size}{\PDFPC@fontsize}
   \immediate\write\pdfpcnotesfile{[notes]}
 }
 % define a # http://tex.stackexchange.com/a/37757/10327