Update to v0.1.2 with HTML support

moved posterdown_pdf to posterdown_latex, added posterdown_html, added to ReadME, added ciation file. added @shedimus  as contributor in Description file.
diff --git a/NAMESPACE b/NAMESPACE
index 8966494..1284ebc 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,6 +1,7 @@
 # Generated by roxygen2: do not edit by hand
 
 export(posterdown_html)
+export(posterdown_latex)
 export(posterdown_pdf)
 import(bookdown)
 import(ggplot2)